build: allow the user to specify the ICU paths for compilation
build: allow the user to specify the ICU paths for compilation

This brings the default compilation model more in line with the
cross-compilation model by allowing the user to specify the ICU UC/I18N
paths when invoking CMake.

karwa/swift,austinzheng/swift,shahmishal/swift,harlanhaskins/swift,benlangmuir/swift,airspeedswift/swift,shajrawi/swift,tkremenek/swift,danielmartin/swift,sschiau/swift,nathawes/swift,parkera/swift,allevato/swift,alblue/swift,danielmartin/swift,jmgc/swift,JGiola/swift,practicalswift/swift,glessard/swift,allevato/swift,shahmishal/swift,nathawes/swift,tjw/swift,tjw/swift,hooman/swift,gribozavr/swift,xedin/swift,apple/swift,benlangmuir/swift,practicalswift/swift,karwa/swift,stephentyrone/swift,danielmartin/swift,lorentey/swift,ahoppen/swift,allevato/swift,huonw/swift,rudkx/swift,gribozavr/swift,JGiola/swift,xedin/swift,sschiau/swift,lorentey/swift,jckarter/swift,rudkx/swift,JGiola/swift,harlanhaskins/swift,harlanhaskins/swift,xedin/swift,tjw/swift,xedin/swift,jmgc/swift,shahmishal/swift,shajrawi/swift,shahmishal/swift,natecook1000/swift,xwu/swift,roambotics/swift,xedin/swift,jmgc/swift,xwu/swift,nathawes/swift,harlanhaskins/swift,tkremenek/swift,apple/swift,hooman/swift,ahoppen/swift,brentdax/swift,brentdax/swift,karwa/swift,austinzheng/swift,practicalswift/swift,amraboelela/swift,aschwaighofer/swift,parkera/swift,stephentyrone/swift,sschiau/swift,xwu/swift,apple/swift,jckarter/swift,jmgc/swift,huonw/swift,parkera/swift,parkera/swift,lorentey/swift,lorentey/swift,austinzheng/swift,jopamer/swift,rudkx/swift,stephentyrone/swift,stephentyrone/swift,allevato/swift,aschwaighofer/swift,brentdax/swift,roambotics/swift,rudkx/swift,aschwaighofer/swift,gregomni/swift,shajrawi/swift,lorentey/swift,glessard/swift,danielmartin/swift,tjw/swift,danielmartin/swift,airspeedswift/swift,jopamer/swift,airspeedswift/swift,jckarter/swift,gribozavr/swift,gribozavr/swift,xwu/swift,airspeedswift/swift,parkera/swift,amraboelela/swift,benlangmuir/swift,jmgc/swift,aschwaighofer/swift,jckarter/swift,alblue/swift,jmgc/swift,nathawes/swift,danielmartin/swift,tjw/swift,karwa/swift,austinzheng/swift,alblue/swift,jopamer/swift,gribozavr/swift,glessard/swift,sschiau/swift,jckarter/swift,lorentey/swift,roambotics/swift,xwu/swift,practicalswift/swift,shahmishal/swift,devincoughlin/swift,gregomni/swift,glessard/swift,aschwaighofer/swift,devincoughlin/swift,tjw/swift,shahmishal/swift,xedin/swift,atrick/swift,karwa/swift,devincoughlin/swift,jopamer/swift,parkera/swift,JGiola/swift,austinzheng/swift,nathawes/swift,glessard/swift,brentdax/swift,lorentey/swift,benlangmuir/swift,atrick/swift,jopamer/swift,huonw/swift,sschiau/swift,tkremenek/swift,parkera/swift,amraboelela/swift,alblue/swift,amraboelela/swift,glessard/swift,gregomni/swift,practicalswift/swift,devincoughlin/swift,harlanhaskins/swift,gregomni/swift,sschiau/swift,shajrawi/swift,shajrawi/swift,ahoppen/swift,xedin/swift,xedin/swift,harlanhaskins/swift,parkera/swift,airspeedswift/swift,stephentyrone/swift,hooman/swift,atrick/swift,ahoppen/swift,benlangmuir/swift,alblue/swift,CodaFi/swift,roambotics/swift,tkremenek/swift,JGiola/swift,CodaFi/swift,natecook1000/swift,shahmishal/swift,tjw/swift,rudkx/swift,natecook1000/swift,natecook1000/swift,benlangmuir/swift,shahmishal/swift,natecook1000/swift,practicalswift/swift,brentdax/swift,tkremenek/swift,gribozavr/swift,CodaFi/swift,aschwaighofer/swift,natecook1000/swift,brentdax/swift,practicalswift/swift,rudkx/swift,natecook1000/swift,ahoppen/swift,devincoughlin/swift,huonw/swift,gregomni/swift,jmgc/swift,allevato/swift,jopamer/swift,atrick/swift,tkremenek/swift,hooman/swift,atrick/swift,sschiau/swift,nathawes/swift,devincoughlin/swift,jopamer/swift,tkremenek/swift,CodaFi/swift,airspeedswift/swift,airspeedswift/swift,shajrawi/swift,huonw/swift,CodaFi/swift,nathawes/swift,atrick/swift,practicalswift/swift,sschiau/swift,gribozavr/swift,roambotics/swift,karwa/swift,gregomni/swift,shajrawi/swift,amraboelela/swift,gribozavr/swift,apple/swift,JGiola/swift,alblue/swift,jckarter/swift,allevato/swift,karwa/swift,lorentey/swift,xwu/swift,amraboelela/swift,stephentyrone/swift,allevato/swift,austinzheng/swift,hooman/swift,CodaFi/swift,jckarter/swift,devincoughlin/swift,huonw/swift,huonw/swift,stephentyrone/swift,ahoppen/swift,hooman/swift,aschwaighofer/swift,apple/swift,brentdax/swift,amraboelela/swift,CodaFi/swift,hooman/swift,apple/swift,austinzheng/swift,karwa/swift,xwu/swift,roambotics/swift,devincoughlin/swift,shajrawi/swift,harlanhaskins/swift,alblue/swift,danielmartin/swift