build: quote a variable for robustness
build: quote a variable for robustness

If the command fails (like when you update Xcode), the string will be empty,
which CMake will evaluate as a missing parameter.  Quote the string to be more
robust in such a case.

danielmartin/swift,danielmartin/swift,brentdax/swift,nathawes/swift,lorentey/swift,stephentyrone/swift,stephentyrone/swift,harlanhaskins/swift,glessard/swift,rudkx/swift,apple/swift,parkera/swift,amraboelela/swift,lorentey/swift,roambotics/swift,amraboelela/swift,xwu/swift,brentdax/swift,rudkx/swift,hooman/swift,atrick/swift,tkremenek/swift,rudkx/swift,hooman/swift,xwu/swift,shahmishal/swift,CodaFi/swift,nathawes/swift,xedin/swift,benlangmuir/swift,airspeedswift/swift,lorentey/swift,xwu/swift,glessard/swift,xedin/swift,rudkx/swift,glessard/swift,practicalswift/swift,tkremenek/swift,gregomni/swift,airspeedswift/swift,apple/swift,ahoppen/swift,amraboelela/swift,sschiau/swift,aschwaighofer/swift,devincoughlin/swift,brentdax/swift,austinzheng/swift,karwa/swift,danielmartin/swift,shahmishal/swift,parkera/swift,ahoppen/swift,CodaFi/swift,danielmartin/swift,parkera/swift,xedin/swift,atrick/swift,hooman/swift,atrick/swift,lorentey/swift,benlangmuir/swift,devincoughlin/swift,devincoughlin/swift,aschwaighofer/swift,jmgc/swift,roambotics/swift,amraboelela/swift,lorentey/swift,sschiau/swift,benlangmuir/swift,gribozavr/swift,CodaFi/swift,aschwaighofer/swift,danielmartin/swift,shajrawi/swift,harlanhaskins/swift,JGiola/swift,gregomni/swift,airspeedswift/swift,allevato/swift,airspeedswift/swift,JGiola/swift,amraboelela/swift,tkremenek/swift,stephentyrone/swift,xedin/swift,gribozavr/swift,gregomni/swift,shajrawi/swift,hooman/swift,karwa/swift,stephentyrone/swift,ahoppen/swift,sschiau/swift,gribozavr/swift,lorentey/swift,xedin/swift,roambotics/swift,practicalswift/swift,gribozavr/swift,gregomni/swift,gribozavr/swift,benlangmuir/swift,hooman/swift,jckarter/swift,gregomni/swift,brentdax/swift,jmgc/swift,jmgc/swift,gribozavr/swift,glessard/swift,devincoughlin/swift,parkera/swift,austinzheng/swift,CodaFi/swift,sschiau/swift,xwu/swift,stephentyrone/swift,apple/swift,karwa/swift,alblue/swift,tkremenek/swift,harlanhaskins/swift,aschwaighofer/swift,jckarter/swift,ahoppen/swift,stephentyrone/swift,xedin/swift,karwa/swift,jckarter/swift,shahmishal/swift,xedin/swift,apple/swift,hooman/swift,alblue/swift,austinzheng/swift,xedin/swift,CodaFi/swift,harlanhaskins/swift,brentdax/swift,shajrawi/swift,devincoughlin/swift,shahmishal/swift,sschiau/swift,nathawes/swift,nathawes/swift,roambotics/swift,jckarter/swift,allevato/swift,danielmartin/swift,aschwaighofer/swift,shajrawi/swift,nathawes/swift,harlanhaskins/swift,shahmishal/swift,atrick/swift,JGiola/swift,allevato/swift,nathawes/swift,devincoughlin/swift,jmgc/swift,allevato/swift,allevato/swift,gribozavr/swift,benlangmuir/swift,practicalswift/swift,airspeedswift/swift,lorentey/swift,austinzheng/swift,alblue/swift,tkremenek/swift,karwa/swift,shahmishal/swift,parkera/swift,shajrawi/swift,parkera/swift,lorentey/swift,harlanhaskins/swift,shahmishal/swift,apple/swift,glessard/swift,karwa/swift,benlangmuir/swift,alblue/swift,shajrawi/swift,roambotics/swift,austinzheng/swift,austinzheng/swift,hooman/swift,karwa/swift,allevato/swift,practicalswift/swift,devincoughlin/swift,shahmishal/swift,xwu/swift,brentdax/swift,airspeedswift/swift,sschiau/swift,JGiola/swift,danielmartin/swift,alblue/swift,shajrawi/swift,gregomni/swift,xwu/swift,practicalswift/swift,amraboelela/swift,austinzheng/swift,practicalswift/swift,sschiau/swift,CodaFi/swift,JGiola/swift,ahoppen/swift,parkera/swift,aschwaighofer/swift,nathawes/swift,practicalswift/swift,shajrawi/swift,jckarter/swift,tkremenek/swift,rudkx/swift,glessard/swift,jmgc/swift,devincoughlin/swift,karwa/swift,allevato/swift,parkera/swift,alblue/swift,xwu/swift,alblue/swift,ahoppen/swift,brentdax/swift,atrick/swift,jmgc/swift,airspeedswift/swift,atrick/swift,gribozavr/swift,amraboelela/swift,roambotics/swift,jckarter/swift,sschiau/swift,practicalswift/swift,apple/swift,aschwaighofer/swift,harlanhaskins/swift,stephentyrone/swift,tkremenek/swift,jckarter/swift,rudkx/swift,CodaFi/swift,jmgc/swift,JGiola/swift