Fix build break caused by r1612

- Don't generate windows environment files on non-Windows
- Don't use `where` to locate cl.exe -- apparently some bots
  are XP?

TBR=thakis@chromium.org

Review URL: https://codereview.chromium.org/13870021

git-svn-id: http://gyp.googlecode.com/svn/trunk@1614 78cadc50-ecff-11dd-a971-7dbc132099af