build: treat ICU includes as system headers
build: treat ICU includes as system headers

These headers should be treated as system headers as we do not control them.

Use target_include_directories so that we can use the `SYSTEM` option to
indicate that they are system headers.  It also simplifies the invocation by not
directly modifying the underlying property.

gmilos/swift,gottesmm/swift,milseman/swift,aschwaighofer/swift,therealbnut/swift,hughbe/swift,benlangmuir/swift,xwu/swift,jopamer/swift,devincoughlin/swift,allevato/swift,shajrawi/swift,hooman/swift,zisko/swift,aschwaighofer/swift,uasys/swift,gottesmm/swift,tkremenek/swift,tkremenek/swift,hooman/swift,alblue/swift,austinzheng/swift,atrick/swift,lorentey/swift,xedin/swift,stephentyrone/swift,karwa/swift,huonw/swift,practicalswift/swift,tjw/swift,harlanhaskins/swift,hooman/swift,glessard/swift,hughbe/swift,tjw/swift,nathawes/swift,amraboelela/swift,shajrawi/swift,uasys/swift,gregomni/swift,devincoughlin/swift,parkera/swift,harlanhaskins/swift,jmgc/swift,amraboelela/swift,brentdax/swift,danielmartin/swift,austinzheng/swift,practicalswift/swift,calebd/swift,therealbnut/swift,jopamer/swift,alblue/swift,Jnosh/swift,codestergit/swift,tinysun212/swift-windows,hooman/swift,therealbnut/swift,xwu/swift,swiftix/swift,jmgc/swift,djwbrown/swift,zisko/swift,deyton/swift,xwu/swift,JGiola/swift,harlanhaskins/swift,benlangmuir/swift,danielmartin/swift,brentdax/swift,jckarter/swift,parkera/swift,xedin/swift,danielmartin/swift,JaSpa/swift,djwbrown/swift,frootloops/swift,natecook1000/swift,xedin/swift,hughbe/swift,zisko/swift,jmgc/swift,roambotics/swift,zisko/swift,JaSpa/swift,tjw/swift,jckarter/swift,gottesmm/swift,shahmishal/swift,xedin/swift,brentdax/swift,bitjammer/swift,codestergit/swift,uasys/swift,aschwaighofer/swift,hooman/swift,nathawes/swift,nathawes/swift,amraboelela/swift,hooman/swift,roambotics/swift,practicalswift/swift,JaSpa/swift,lorentey/swift,lorentey/swift,jopamer/swift,lorentey/swift,manavgabhawala/swift,devincoughlin/swift,lorentey/swift,Jnosh/swift,ahoppen/swift,JaSpa/swift,milseman/swift,CodaFi/swift,bitjammer/swift,felix91gr/swift,codestergit/swift,JGiola/swift,arvedviehweger/swift,OscarSwanros/swift,stephentyrone/swift,shajrawi/swift,karwa/swift,return/swift,benlangmuir/swift,felix91gr/swift,tinysun212/swift-windows,return/swift,allevato/swift,brentdax/swift,sschiau/swift,airspeedswift/swift,OscarSwanros/swift,danielmartin/swift,zisko/swift,gribozavr/swift,jtbandes/swift,therealbnut/swift,djwbrown/swift,deyton/swift,calebd/swift,gribozavr/swift,calebd/swift,rudkx/swift,apple/swift,djwbrown/swift,rudkx/swift,felix91gr/swift,arvedviehweger/swift,jopamer/swift,xedin/swift,atrick/swift,shahmishal/swift,jtbandes/swift,sschiau/swift,therealbnut/swift,practicalswift/swift,JaSpa/swift,uasys/swift,Jnosh/swift,tkremenek/swift,stephentyrone/swift,milseman/swift,lorentey/swift,tkremenek/swift,felix91gr/swift,ahoppen/swift,return/swift,nathawes/swift,alblue/swift,tinysun212/swift-windows,alblue/swift,jtbandes/swift,Jnosh/swift,karwa/swift,roambotics/swift,hughbe/swift,milseman/swift,arvedviehweger/swift,gregomni/swift,sschiau/swift,djwbrown/swift,tkremenek/swift,roambotics/swift,jckarter/swift,devincoughlin/swift,bitjammer/swift,natecook1000/swift,sschiau/swift,gottesmm/swift,deyton/swift,huonw/swift,CodaFi/swift,shahmishal/swift,calebd/swift,benlangmuir/swift,jopamer/swift,tinysun212/swift-windows,devincoughlin/swift,arvedviehweger/swift,glessard/swift,felix91gr/swift,allevato/swift,deyton/swift,JGiola/swift,parkera/swift,gmilos/swift,practicalswift/swift,xedin/swift,return/swift,parkera/swift,tardieu/swift,shahmishal/swift,gregomni/swift,amraboelela/swift,swiftix/swift,glessard/swift,jckarter/swift,frootloops/swift,huonw/swift,swiftix/swift,gribozavr/swift,ahoppen/swift,frootloops/swift,sschiau/swift,xwu/swift,atrick/swift,Jnosh/swift,frootloops/swift,natecook1000/swift,shahmishal/swift,aschwaighofer/swift,stephentyrone/swift,shahmishal/swift,nathawes/swift,roambotics/swift,gribozavr/swift,tjw/swift,practicalswift/swift,huonw/swift,gottesmm/swift,parkera/swift,aschwaighofer/swift,shajrawi/swift,hughbe/swift,hooman/swift,hughbe/swift,codestergit/swift,harlanhaskins/swift,stephentyrone/swift,gmilos/swift,JGiola/swift,karwa/swift,benlangmuir/swift,jmgc/swift,allevato/swift,manavgabhawala/swift,devincoughlin/swift,calebd/swift,uasys/swift,karwa/swift,alblue/swift,manavgabhawala/swift,austinzheng/swift,OscarSwanros/swift,gribozavr/swift,parkera/swift,jtbandes/swift,jtbandes/swift,felix91gr/swift,swiftix/swift,stephentyrone/swift,airspeedswift/swift,gmilos/swift,sschiau/swift,return/swift,arvedviehweger/swift,austinzheng/swift,bitjammer/swift,tkremenek/swift,amraboelela/swift,zisko/swift,glessard/swift,gregomni/swift,uasys/swift,huonw/swift,frootloops/swift,JaSpa/swift,huonw/swift,codestergit/swift,shahmishal/swift,airspeedswift/swift,frootloops/swift,shahmishal/swift,tardieu/swift,karwa/swift,jtbandes/swift,tkremenek/swift,gmilos/swift,danielmartin/swift,gribozavr/swift,apple/swift,parkera/swift,harlanhaskins/swift,harlanhaskins/swift,allevato/swift,felix91gr/swift,natecook1000/swift,therealbnut/swift,apple/swift,ahoppen/swift,tardieu/swift,bitjammer/swift,austinzheng/swift,apple/swift,shajrawi/swift,gribozavr/swift,atrick/swift,gregomni/swift,gregomni/swift,arvedviehweger/swift,airspeedswift/swift,milseman/swift,atrick/swift,xwu/swift,brentdax/swift,milseman/swift,shajrawi/swift,rudkx/swift,allevato/swift,roambotics/swift,jopamer/swift,milseman/swift,gottesmm/swift,natecook1000/swift,xwu/swift,tjw/swift,huonw/swift,tinysun212/swift-windows,airspeedswift/swift,zisko/swift,devincoughlin/swift,therealbnut/swift,amraboelela/swift,gmilos/swift,xwu/swift,amraboelela/swift,OscarSwanros/swift,swiftix/swift,jckarter/swift,hughbe/swift,ahoppen/swift,tinysun212/swift-windows,manavgabhawala/swift,shajrawi/swift,manavgabhawala/swift,JGiola/swift,jmgc/swift,tjw/swift,OscarSwanros/swift,Jnosh/swift,jtbandes/swift,uasys/swift,CodaFi/swift,airspeedswift/swift,shajrawi/swift,glessard/swift,benlangmuir/swift,rudkx/swift,austinzheng/swift,bitjammer/swift,xedin/swift,gmilos/swift,brentdax/swift,JGiola/swift,codestergit/swift,apple/swift,sschiau/swift,natecook1000/swift,tardieu/swift,brentdax/swift,aschwaighofer/swift,jmgc/swift,harlanhaskins/swift,codestergit/swift,alblue/swift,CodaFi/swift,djwbrown/swift,apple/swift,gribozavr/swift,JaSpa/swift,swiftix/swift,OscarSwanros/swift,stephentyrone/swift,aschwaighofer/swift,lorentey/swift,CodaFi/swift,jopamer/swift,tjw/swift,tardieu/swift,devincoughlin/swift,jmgc/swift,allevato/swift,bitjammer/swift,Jnosh/swift,calebd/swift,airspeedswift/swift,nathawes/swift,austinzheng/swift,lorentey/swift,natecook1000/swift,alblue/swift,swiftix/swift,frootloops/swift,OscarSwanros/swift,rudkx/swift,djwbrown/swift,arvedviehweger/swift,karwa/swift,tardieu/swift,calebd/swift,manavgabhawala/swift,jckarter/swift,CodaFi/swift,ahoppen/swift,danielmartin/swift,return/swift,deyton/swift,jckarter/swift,manavgabhawala/swift,sschiau/swift,xedin/swift,practicalswift/swift,deyton/swift,CodaFi/swift,glessard/swift,return/swift,tinysun212/swift-windows,gottesmm/swift,practicalswift/swift,parkera/swift,rudkx/swift,karwa/swift,tardieu/swift,atrick/swift,danielmartin/swift,deyton/swift,nathawes/swift