Set -isysroot to an available SDK on OS X.
Set -isysroot to an available SDK on OS X.

Previously cling was basically unusable on OS X unless the CommandLineTools were
installed. Now if XCode is installed the system headers from an SDK can be used.

Signed-off-by: Vassil Vassilev <0b291c0bdb4e22c8b7705ceedf8e168ff73f20e8@cern.ch>

davidlt/root,georgtroska/root,pspe/root,simonpf/root,abhinavmoudgil95/root,BerserkerTroll/root,gbitzes/root,zzxuanyuan/root,bbockelm/root,gganis/root,root-mirror/root,satyarth934/root,thomaskeck/root,davidlt/root,pspe/root,mhuwiler/rootauto,pspe/root,Y--/root,BerserkerTroll/root,lgiommi/root,pspe/root,lgiommi/root,mhuwiler/rootauto,davidlt/root,Y--/root,gbitzes/root,zzxuanyuan/root,karies/root,Y--/root,thomaskeck/root,simonpf/root,karies/root,davidlt/root,satyarth934/root,agarciamontoro/root,agarciamontoro/root,gganis/root,olifre/root,abhinavmoudgil95/root,gbitzes/root,agarciamontoro/root,olifre/root,lgiommi/root,georgtroska/root,mhuwiler/rootauto,simonpf/root,olifre/root,olifre/root,root-mirror/root,Y--/root,agarciamontoro/root,root-mirror/root,satyarth934/root,zzxuanyuan/root-compressor-dummy,lgiommi/root,karies/root,karies/root,satyarth934/root,zzxuanyuan/root-compressor-dummy,veprbl/root,davidlt/root,davidlt/root,bbockelm/root,lgiommi/root,karies/root,lgiommi/root,Y--/root,karies/root,karies/root,thomaskeck/root,zzxuanyuan/root-compressor-dummy,buuck/root,BerserkerTroll/root,beniz/root,root-mirror/root,root-mirror/root,bbockelm/root,Y--/root,veprbl/root,satyarth934/root,bbockelm/root,Y--/root,buuck/root,BerserkerTroll/root,abhinavmoudgil95/root,lgiommi/root,beniz/root,gganis/root,zzxuanyuan/root-compressor-dummy,olifre/root,mhuwiler/rootauto,simonpf/root,thomaskeck/root,BerserkerTroll/root,gbitzes/root,veprbl/root,satyarth934/root,buuck/root,pspe/root,karies/root,lgiommi/root,mhuwiler/rootauto,gbitzes/root,buuck/root,buuck/root,beniz/root,zzxuanyuan/root-compressor-dummy,georgtroska/root,abhinavmoudgil95/root,abhinavmoudgil95/root,karies/root,thomaskeck/root,zzxuanyuan/root,veprbl/root,davidlt/root,root-mirror/root,agarciamontoro/root,BerserkerTroll/root,georgtroska/root,bbockelm/root,root-mirror/root,georgtroska/root,pspe/root,mhuwiler/rootauto,olifre/root,thomaskeck/root,mhuwiler/rootauto,beniz/root,zzxuanyuan/root-compressor-dummy,abhinavmoudgil95/root,zzxuanyuan/root-compressor-dummy,BerserkerTroll/root,gbitzes/root,pspe/root,zzxuanyuan/root,Y--/root,olifre/root,zzxuanyuan/root,gbitzes/root,bbockelm/root,veprbl/root,beniz/root,georgtroska/root,agarciamontoro/root,agarciamontoro/root,veprbl/root,buuck/root,buuck/root,zzxuanyuan/root-compressor-dummy,zzxuanyuan/root,thomaskeck/root,georgtroska/root,bbockelm/root,BerserkerTroll/root,buuck/root,karies/root,olifre/root,bbockelm/root,gganis/root,georgtroska/root,zzxuanyuan/root-compressor-dummy,gbitzes/root,simonpf/root,georgtroska/root,karies/root,pspe/root,gganis/root,beniz/root,simonpf/root,bbockelm/root,davidlt/root,gganis/root,olifre/root,satyarth934/root,veprbl/root,zzxuanyuan/root,root-mirror/root,abhinavmoudgil95/root,simonpf/root,buuck/root,veprbl/root,zzxuanyuan/root-compressor-dummy,gganis/root,olifre/root,pspe/root,mhuwiler/rootauto,beniz/root,BerserkerTroll/root,Y--/root,abhinavmoudgil95/root,gganis/root,satyarth934/root,satyarth934/root,beniz/root,satyarth934/root,abhinavmoudgil95/root,gganis/root,mhuwiler/rootauto,thomaskeck/root,mhuwiler/rootauto,georgtroska/root,lgiommi/root,Y--/root,simonpf/root,gbitzes/root,abhinavmoudgil95/root,zzxuanyuan/root,davidlt/root,zzxuanyuan/root,BerserkerTroll/root,davidlt/root,agarciamontoro/root,veprbl/root,thomaskeck/root,simonpf/root,buuck/root,abhinavmoudgil95/root,gganis/root,bbockelm/root,root-mirror/root,root-mirror/root,thomaskeck/root,agarciamontoro/root,simonpf/root,zzxuanyuan/root-compressor-dummy,Y--/root,olifre/root,zzxuanyuan/root,BerserkerTroll/root,gbitzes/root,gganis/root,simonpf/root,pspe/root,mhuwiler/rootauto,gbitzes/root,root-mirror/root,pspe/root,satyarth934/root,davidlt/root,beniz/root,lgiommi/root,zzxuanyuan/root,buuck/root,agarciamontoro/root,beniz/root,georgtroska/root,beniz/root,zzxuanyuan/root,veprbl/root,bbockelm/root,agarciamontoro/root,lgiommi/root,veprbl/root