ninja win: Remove obsolete comment, and make sure cc/cxx are always overridden

Builds on discussion in https://codereview.chromium.org/321813002/.

Make sure that cc_host and cxx_host are also set to a valid compiler,
which fixes the cflags test.

R=yukawa@chromium.org
BUG=chromium:233985

Review URL: https://codereview.chromium.org/324383004

git-svn-id: http://gyp.googlecode.com/svn/trunk@1933 78cadc50-ecff-11dd-a971-7dbc132099af
1 file changed