Ninja generator: support using clang-cl for Windows 

If CC is set to 'clang-cl' in make_global_settings, use that 
as the compiler instead of cl.exe from the environment. 

BUG=chromium:82385
Review URL: https://codereview.chromium.org/136113016/

Patch from Hans Wennborg <hans@chromium.org>


git-svn-id: http://gyp.googlecode.com/svn/trunk@1846 78cadc50-ecff-11dd-a971-7dbc132099af