Fix discovery of Darwin SDKs
Fix discovery of Darwin SDKs

The current method doesn't work with Xcode 4 and up, since they no
longer have a $DEVELOPER_DIR/SDKs directory. Using xcrun and xcodebuild
works all the way back to Xcode 3 on OS X 10.6 Snow Leopard, if not
earlier.

Change-Id: I7126f2fb4a8f1d6e46f921e70bbd090f00ce3d36

shareefalis/libvpx,GrokImageCompression/aom,stewnorriss/libvpx,shareefalis/libvpx,webmproject/libvpx,matanbs/webm.libvpx,Distrotech/libvpx,altogother/webm.libvpx,felipebetancur/libvpx,reimaginemedia/webm.libvpx,pcwalton/libvpx,zofuthan/libvpx,VTCSecureLLC/libvpx,GrokImageCompression/aom,matanbs/webm.libvpx,Acidburn0zzz/webm.libvpx,Distrotech/libvpx,mwgoldsmith/vpx,goodleixiao/vpx,webmproject/libvpx,reimaginemedia/webm.libvpx,running770/libvpx,mwgoldsmith/libvpx,ittiamvpx/libvpx-1,jdm/libvpx,stewnorriss/libvpx,abwiz0086/webm.libvpx,running770/libvpx,jmvalin/aom,shareefalis/libvpx,kleopatra999/webm.libvpx,ShiftMediaProject/libvpx,jdm/libvpx,smarter/aom,mbebenita/aom,Acidburn0zzz/webm.libvpx,Maria1099/webm.libvpx,GrokImageCompression/aom,lyx2014/libvpx_c,iniwf/webm.libvpx,kim42083/webm.libvpx,webmproject/libvpx,mwgoldsmith/vpx,mbebenita/aom,kalli123/webm.libvpx,Topopiccione/libvpx,ShiftMediaProject/libvpx,GrokImageCompression/aom,mbebenita/aom,stewnorriss/libvpx,Acidburn0zzz/webm.libvpx,openpeer/libvpx_new,altogother/webm.libvpx,felipebetancur/libvpx,jmvalin/aom,mwgoldsmith/libvpx,lyx2014/libvpx_c,pcwalton/libvpx,openpeer/libvpx_new,Suvarna1488/webm.libvpx,abwiz0086/webm.libvpx,felipebetancur/libvpx,liqianggao/libvpx,VTCSecureLLC/libvpx,jdm/libvpx,Topopiccione/libvpx,zofuthan/libvpx,iniwf/webm.libvpx,Acidburn0zzz/webm.libvpx,thdav/aom,kim42083/webm.libvpx,openpeer/libvpx_new,ittiamvpx/libvpx-1,pcwalton/libvpx,ittiamvpx/libvpx-1,running770/libvpx,jmvalin/aom,gshORTON/webm.libvpx,Topopiccione/libvpx,shacklettbp/aom,luctrudeau/aom,VTCSecureLLC/libvpx,reimaginemedia/webm.libvpx,lyx2014/libvpx_c,kalli123/webm.libvpx,gshORTON/webm.libvpx,iniwf/webm.libvpx,matanbs/webm.libvpx,smarter/aom,shacklettbp/aom,thdav/aom,mbebenita/aom,openpeer/libvpx_new,abwiz0086/webm.libvpx,gshORTON/webm.libvpx,shyamalschandra/libvpx,goodleixiao/vpx,kalli123/webm.libvpx,liqianggao/libvpx,mbebenita/aom,thdav/aom,jmvalin/aom,goodleixiao/vpx,thdav/aom,stewnorriss/libvpx,ittiamvpx/libvpx-1,reimaginemedia/webm.libvpx,Suvarna1488/webm.libvpx,altogother/webm.libvpx,webmproject/libvpx,matanbs/webm.libvpx,VTCSecureLLC/libvpx,ittiamvpx/libvpx-1,kleopatra999/webm.libvpx,ittiamvpx/libvpx-1,shyamalschandra/libvpx,luctrudeau/aom,Suvarna1488/webm.libvpx,shacklettbp/aom,reimaginemedia/webm.libvpx,jdm/libvpx,running770/libvpx,Distrotech/libvpx,kleopatra999/webm.libvpx,jmvalin/aom,webmproject/libvpx,altogother/webm.libvpx,jmvalin/aom,VTCSecureLLC/libvpx,lyx2014/libvpx_c,gshORTON/webm.libvpx,shareefalis/libvpx,mwgoldsmith/vpx,stewnorriss/libvpx,felipebetancur/libvpx,mwgoldsmith/libvpx,mwgoldsmith/vpx,Maria1099/webm.libvpx,goodleixiao/vpx,kleopatra999/webm.libvpx,mwgoldsmith/libvpx,kleopatra999/webm.libvpx,kalli123/webm.libvpx,reimaginemedia/webm.libvpx,Acidburn0zzz/webm.libvpx,abwiz0086/webm.libvpx,liqianggao/libvpx,pcwalton/libvpx,liqianggao/libvpx,Distrotech/libvpx,shyamalschandra/libvpx,felipebetancur/libvpx,stewnorriss/libvpx,goodleixiao/vpx,Maria1099/webm.libvpx,pcwalton/libvpx,shareefalis/libvpx,matanbs/webm.libvpx,gshORTON/webm.libvpx,Suvarna1488/webm.libvpx,luctrudeau/aom,ShiftMediaProject/libvpx,shareefalis/libvpx,luctrudeau/aom,zofuthan/libvpx,Topopiccione/libvpx,iniwf/webm.libvpx,kim42083/webm.libvpx,jacklicn/webm.libvpx,shyamalschandra/libvpx,kalli123/webm.libvpx,mwgoldsmith/vpx,GrokImageCompression/aom,jacklicn/webm.libvpx,pcwalton/libvpx,Maria1099/webm.libvpx,running770/libvpx,Acidburn0zzz/webm.libvpx,luctrudeau/aom,mbebenita/aom,luctrudeau/aom,zofuthan/libvpx,running770/libvpx,smarter/aom,goodleixiao/vpx,iniwf/webm.libvpx,lyx2014/libvpx_c,Maria1099/webm.libvpx,smarter/aom,iniwf/webm.libvpx,mwgoldsmith/libvpx,smarter/aom,VTCSecureLLC/libvpx,shacklettbp/aom,ShiftMediaProject/libvpx,abwiz0086/webm.libvpx,liqianggao/libvpx,Maria1099/webm.libvpx,openpeer/libvpx_new,jacklicn/webm.libvpx,smarter/aom,ShiftMediaProject/libvpx,felipebetancur/libvpx,mbebenita/aom,jacklicn/webm.libvpx,mbebenita/aom,matanbs/webm.libvpx,thdav/aom,jacklicn/webm.libvpx,jdm/libvpx,Suvarna1488/webm.libvpx,shyamalschandra/libvpx,zofuthan/libvpx,openpeer/libvpx_new,Distrotech/libvpx,kim42083/webm.libvpx,kleopatra999/webm.libvpx,altogother/webm.libvpx,GrokImageCompression/aom,jacklicn/webm.libvpx,Topopiccione/libvpx,kim42083/webm.libvpx,kalli123/webm.libvpx,altogother/webm.libvpx,mwgoldsmith/vpx,thdav/aom,jdm/libvpx,shacklettbp/aom,shyamalschandra/libvpx,zofuthan/libvpx,Topopiccione/libvpx,lyx2014/libvpx_c,gshORTON/webm.libvpx,liqianggao/libvpx,shacklettbp/aom,webmproject/libvpx,kim42083/webm.libvpx,mwgoldsmith/libvpx,Suvarna1488/webm.libvpx,abwiz0086/webm.libvpx,Distrotech/libvpx,mbebenita/aom