ninja windows: keep cc as something valid for c99conv

Keep "cc = cl.exe" around because c99conv uses it to try to find what it
should use to preprocess. It should probably be passed this somehow else
instead.

R=thakis@chromium.org
BUG=233985

Review URL: https://codereview.chromium.org/14145005

git-svn-id: http://gyp.googlecode.com/svn/trunk@1620 78cadc50-ecff-11dd-a971-7dbc132099af