Add support for make_global_settings which allows setting  CC and friends from gyp. 
Add support for make_global_settings which allows setting 
CC and friends from gyp. 

The motivation is that 

GYP_DEFINES=clang=1 build/gyp_chromium && make 

should be enough to build with clang. 

Demo: http://codereview.chromium.org/7694023 
(Also see http://codereview.chromium.org/7619001 )
Review URL: http://codereview.chromium.org/7696021/


old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google,old8xp/gyp_from_google