ninja: make cross-compilation use $CC/$CXX for the target compiler 
ninja: make cross-compilation use $CC/$CXX for the target compiler 

This new patch should fix the mac bot issue.

Took over from http://codereview.chromium.org/9649016/ 

BUG=140900
TEST=set CC/CXX to cross-compiler, set GYP_CROSSCOMPILE to 1, run gyp, ninja chrome.



csulmone/gyp,csulmone/gyp,csulmone/gyp,csulmone/gyp