make: Let CC_host, CXX_host env vars win over make_global_settings keys.

This is what's happening in the ninja generator, and what's happening in the
make generator for CC / CXX too.

BUG=chromium:326411
Review URL: https://codereview.chromium.org/117823012/



git-svn-id: http://gyp.googlecode.com/svn/trunk@1824 78cadc50-ecff-11dd-a971-7dbc132099af