cmake: unbreak the usecase of cross-compiling the compiler while disabling the standard library
cmake: unbreak the usecase of cross-compiling the compiler while
disabling the standard library

In almost all cases, CMake will happily operate on undefined variables,
treating them as empty, but for some reason list(REMOVE_ITEM) is an
exception.

Swift SVN r25835

roambotics/swift,ken0nek/swift,JaSpa/swift,arvedviehweger/swift,kusl/swift,parkera/swift,emilstahl/swift,apple/swift,karwa/swift,hooman/swift,emilstahl/swift,swiftix/swift.old,bitjammer/swift,karwa/swift,shajrawi/swift,Jnosh/swift,gottesmm/swift,JaSpa/swift,jtbandes/swift,slavapestov/swift,lorentey/swift,IngmarStein/swift,roambotics/swift,gribozavr/swift,ben-ng/swift,jopamer/swift,felix91gr/swift,emilstahl/swift,bitjammer/swift,adrfer/swift,gregomni/swift,practicalswift/swift,SwiftAndroid/swift,kentya6/swift,khizkhiz/swift,allevato/swift,SwiftAndroid/swift,sschiau/swift,therealbnut/swift,swiftix/swift.old,swiftix/swift.old,slavapestov/swift,shahmishal/swift,kentya6/swift,manavgabhawala/swift,aschwaighofer/swift,zisko/swift,SwiftAndroid/swift,Ivacker/swift,khizkhiz/swift,tjw/swift,ken0nek/swift,nathawes/swift,jtbandes/swift,alblue/swift,gribozavr/swift,emilstahl/swift,jckarter/swift,parkera/swift,CodaFi/swift,shahmishal/swift,swiftix/swift.old,brentdax/swift,uasys/swift,therealbnut/swift,ben-ng/swift,jopamer/swift,SwiftAndroid/swift,austinzheng/swift,calebd/swift,Jnosh/swift,jopamer/swift,ken0nek/swift,LeoShimonaka/swift,airspeedswift/swift,practicalswift/swift,sdulal/swift,modocache/swift,xedin/swift,deyton/swift,uasys/swift,danielmartin/swift,slavapestov/swift,JaSpa/swift,OscarSwanros/swift,djwbrown/swift,harlanhaskins/swift,MukeshKumarS/Swift,Ivacker/swift,return/swift,OscarSwanros/swift,jmgc/swift,mightydeveloper/swift,uasys/swift,manavgabhawala/swift,lorentey/swift,SwiftAndroid/swift,shajrawi/swift,cbrentharris/swift,hooman/swift,zisko/swift,codestergit/swift,arvedviehweger/swift,hughbe/swift,IngmarStein/swift,shajrawi/swift,stephentyrone/swift,airspeedswift/swift,KrishMunot/swift,OscarSwanros/swift,brentdax/swift,natecook1000/swift,return/swift,jmgc/swift,alblue/swift,allevato/swift,tjw/swift,shajrawi/swift,khizkhiz/swift,milseman/swift,KrishMunot/swift,emilstahl/swift,stephentyrone/swift,gmilos/swift,tjw/swift,return/swift,amraboelela/swift,arvedviehweger/swift,kentya6/swift,hooman/swift,milseman/swift,gribozavr/swift,kstaring/swift,therealbnut/swift,russbishop/swift,austinzheng/swift,felix91gr/swift,alblue/swift,brentdax/swift,LeoShimonaka/swift,devincoughlin/swift,practicalswift/swift,zisko/swift,slavapestov/swift,huonw/swift,OscarSwanros/swift,danielmartin/swift,hughbe/swift,adrfer/swift,harlanhaskins/swift,shajrawi/swift,ken0nek/swift,johnno1962d/swift,dduan/swift,jckarter/swift,JaSpa/swift,glessard/swift,arvedviehweger/swift,parkera/swift,codestergit/swift,jckarter/swift,SwiftAndroid/swift,johnno1962d/swift,brentdax/swift,brentdax/swift,huonw/swift,tjw/swift,kperryua/swift,xwu/swift,deyton/swift,rudkx/swift,tinysun212/swift-windows,djwbrown/swift,Jnosh/swift,tardieu/swift,amraboelela/swift,roambotics/swift,alblue/swift,ben-ng/swift,emilstahl/swift,allevato/swift,swiftix/swift,huonw/swift,shahmishal/swift,xedin/swift,sdulal/swift,cbrentharris/swift,harlanhaskins/swift,aschwaighofer/swift,jtbandes/swift,atrick/swift,lorentey/swift,return/swift,danielmartin/swift,gottesmm/swift,gmilos/swift,johnno1962d/swift,xwu/swift,tinysun212/swift-windows,glessard/swift,deyton/swift,roambotics/swift,amraboelela/swift,gregomni/swift,sschiau/swift,codestergit/swift,LeoShimonaka/swift,danielmartin/swift,KrishMunot/swift,jmgc/swift,return/swift,manavgabhawala/swift,russbishop/swift,swiftix/swift,deyton/swift,uasys/swift,deyton/swift,practicalswift/swift,apple/swift,JGiola/swift,adrfer/swift,sschiau/swift,dduan/swift,ben-ng/swift,sschiau/swift,apple/swift,modocache/swift,huonw/swift,sdulal/swift,dreamsxin/swift,amraboelela/swift,rudkx/swift,felix91gr/swift,tjw/swift,gottesmm/swift,jtbandes/swift,slavapestov/swift,airspeedswift/swift,danielmartin/swift,practicalswift/swift,frootloops/swift,KrishMunot/swift,mightydeveloper/swift,glessard/swift,arvedviehweger/swift,jckarter/swift,SwiftAndroid/swift,ken0nek/swift,felix91gr/swift,frootloops/swift,bitjammer/swift,russbishop/swift,kstaring/swift,djwbrown/swift,milseman/swift,lorentey/swift,kusl/swift,stephentyrone/swift,calebd/swift,atrick/swift,kusl/swift,natecook1000/swift,tkremenek/swift,shahmishal/swift,ahoppen/swift,johnno1962d/swift,modocache/swift,kperryua/swift,kstaring/swift,gribozavr/swift,CodaFi/swift,kstaring/swift,harlanhaskins/swift,aschwaighofer/swift,hughbe/swift,karwa/swift,nathawes/swift,mightydeveloper/swift,xedin/swift,glessard/swift,karwa/swift,sschiau/swift,practicalswift/swift,JaSpa/swift,Ivacker/swift,shajrawi/swift,huonw/swift,djwbrown/swift,deyton/swift,jopamer/swift,CodaFi/swift,jmgc/swift,jopamer/swift,dduan/swift,sschiau/swift,milseman/swift,alblue/swift,airspeedswift/swift,devincoughlin/swift,atrick/swift,gmilos/swift,jopamer/swift,kentya6/swift,tkremenek/swift,calebd/swift,ben-ng/swift,huonw/swift,xedin/swift,uasys/swift,milseman/swift,IngmarStein/swift,cbrentharris/swift,calebd/swift,sdulal/swift,deyton/swift,russbishop/swift,gottesmm/swift,harlanhaskins/swift,apple/swift,shahmishal/swift,LeoShimonaka/swift,swiftix/swift.old,jckarter/swift,mightydeveloper/swift,tinysun212/swift-windows,austinzheng/swift,tinysun212/swift-windows,tardieu/swift,emilstahl/swift,gribozavr/swift,gmilos/swift,swiftix/swift,tardieu/swift,rudkx/swift,therealbnut/swift,IngmarStein/swift,milseman/swift,harlanhaskins/swift,tardieu/swift,calebd/swift,djwbrown/swift,kperryua/swift,johnno1962d/swift,devincoughlin/swift,swiftix/swift,shajrawi/swift,jtbandes/swift,sdulal/swift,natecook1000/swift,kusl/swift,khizkhiz/swift,practicalswift/swift,cbrentharris/swift,russbishop/swift,allevato/swift,apple/swift,devincoughlin/swift,mightydeveloper/swift,modocache/swift,bitjammer/swift,therealbnut/swift,gregomni/swift,gribozavr/swift,bitjammer/swift,djwbrown/swift,LeoShimonaka/swift,ken0nek/swift,nathawes/swift,JGiola/swift,roambotics/swift,karwa/swift,modocache/swift,tkremenek/swift,Jnosh/swift,brentdax/swift,benlangmuir/swift,calebd/swift,kusl/swift,kentya6/swift,glessard/swift,gregomni/swift,KrishMunot/swift,hughbe/swift,return/swift,JaSpa/swift,mightydeveloper/swift,modocache/swift,benlangmuir/swift,gmilos/swift,gribozavr/swift,milseman/swift,adrfer/swift,austinzheng/swift,johnno1962d/swift,slavapestov/swift,IngmarStein/swift,hooman/swift,airspeedswift/swift,ken0nek/swift,nathawes/swift,Jnosh/swift,swiftix/swift.old,jtbandes/swift,calebd/swift,tardieu/swift,xedin/swift,gottesmm/swift,cbrentharris/swift,nathawes/swift,felix91gr/swift,allevato/swift,felix91gr/swift,jtbandes/swift,Ivacker/swift,arvedviehweger/swift,sdulal/swift,MukeshKumarS/Swift,dduan/swift,LeoShimonaka/swift,natecook1000/swift,tinysun212/swift-windows,kperryua/swift,gottesmm/swift,xedin/swift,huonw/swift,russbishop/swift,felix91gr/swift,cbrentharris/swift,dduan/swift,natecook1000/swift,parkera/swift,zisko/swift,shahmishal/swift,xwu/swift,adrfer/swift,tjw/swift,stephentyrone/swift,Ivacker/swift,kusl/swift,sdulal/swift,manavgabhawala/swift,parkera/swift,bitjammer/swift,uasys/swift,kentya6/swift,austinzheng/swift,JGiola/swift,kperryua/swift,benlangmuir/swift,emilstahl/swift,khizkhiz/swift,practicalswift/swift,devincoughlin/swift,frootloops/swift,codestergit/swift,OscarSwanros/swift,nathawes/swift,apple/swift,bitjammer/swift,JaSpa/swift,frootloops/swift,lorentey/swift,frootloops/swift,gmilos/swift,MukeshKumarS/Swift,shajrawi/swift,kstaring/swift,adrfer/swift,codestergit/swift,gregomni/swift,ahoppen/swift,devincoughlin/swift,dduan/swift,CodaFi/swift,Jnosh/swift,natecook1000/swift,glessard/swift,frootloops/swift,CodaFi/swift,swiftix/swift.old,devincoughlin/swift,codestergit/swift,hughbe/swift,IngmarStein/swift,roambotics/swift,ben-ng/swift,danielmartin/swift,xedin/swift,hughbe/swift,xwu/swift,JGiola/swift,OscarSwanros/swift,zisko/swift,austinzheng/swift,parkera/swift,stephentyrone/swift,nathawes/swift,LeoShimonaka/swift,alblue/swift,sdulal/swift,MukeshKumarS/Swift,atrick/swift,tardieu/swift,ahoppen/swift,xedin/swift,Ivacker/swift,ahoppen/swift,MukeshKumarS/Swift,aschwaighofer/swift,jmgc/swift,manavgabhawala/swift,dduan/swift,cbrentharris/swift,hooman/swift,devincoughlin/swift,ahoppen/swift,mightydeveloper/swift,arvedviehweger/swift,hooman/swift,amraboelela/swift,tkremenek/swift,tkremenek/swift,shahmishal/swift,gottesmm/swift,lorentey/swift,benlangmuir/swift,Ivacker/swift,swiftix/swift.old,xwu/swift,LeoShimonaka/swift,slavapestov/swift,amraboelela/swift,kstaring/swift,airspeedswift/swift,stephentyrone/swift,kperryua/swift,jckarter/swift,tjw/swift,therealbnut/swift,sschiau/swift,MukeshKumarS/Swift,tardieu/swift,aschwaighofer/swift,ahoppen/swift,lorentey/swift,gregomni/swift,khizkhiz/swift,stephentyrone/swift,kperryua/swift,swiftix/swift,karwa/swift,uasys/swift,harlanhaskins/swift,brentdax/swift,allevato/swift,IngmarStein/swift,parkera/swift,kusl/swift,jmgc/swift,austinzheng/swift,tinysun212/swift-windows,jopamer/swift,rudkx/swift,KrishMunot/swift,swiftix/swift,jckarter/swift,danielmartin/swift,kstaring/swift,tkremenek/swift,frootloops/swift,manavgabhawala/swift,aschwaighofer/swift,cbrentharris/swift,hughbe/swift,CodaFi/swift,codestergit/swift,hooman/swift,atrick/swift,zisko/swift,gmilos/swift,russbishop/swift,shahmishal/swift,aschwaighofer/swift,zisko/swift,gribozavr/swift,ben-ng/swift,atrick/swift,manavgabhawala/swift,dreamsxin/swift,OscarSwanros/swift,alblue/swift,benlangmuir/swift,JGiola/swift,allevato/swift,natecook1000/swift,amraboelela/swift,parkera/swift,kusl/swift,swiftix/swift,karwa/swift,MukeshKumarS/Swift,JGiola/swift,xwu/swift,xwu/swift,tkremenek/swift,djwbrown/swift,johnno1962d/swift,Ivacker/swift,Jnosh/swift,kentya6/swift,sschiau/swift,lorentey/swift,rudkx/swift,modocache/swift,kentya6/swift,therealbnut/swift,adrfer/swift,karwa/swift,khizkhiz/swift,benlangmuir/swift,tinysun212/swift-windows,return/swift,KrishMunot/swift,rudkx/swift,airspeedswift/swift,CodaFi/swift,jmgc/swift,mightydeveloper/swift