Make build/gyp_v8 cope with ninja on linux

ninja doesn't support/need the output dir and generator dir flags

BUG=v8:3252
R=machenbach@chromium.org
LOG=n

Review URL: https://codereview.chromium.org/222023004

git-svn-id: http://v8.googlecode.com/svn/branches/bleeding_edge@20462 ce2b1a6d-e550-0410-aec6-3dcde31c8c00

build/gyp_v8

@@ -158,7 +158,8 @@ if __name__ == '__main__':
 
   # Generate for the architectures supported on the given platform.
   gyp_args = list(args)
-  if platform.system() == 'Linux':
+  gyp_generators = os.environ.get('GYP_GENERATORS')
+  if platform.system() == 'Linux' and gyp_generators != 'ninja':
     # Work around for crbug.com/331475.
     for f in glob.glob(os.path.join(v8_root, 'out', 'Makefile.*')):
       os.unlink(f)