Fix _wrapper vars on win.

Workaround for environment vars being uppercase on Windows Python.

Quote the path to cl.exe in cl_x86 and cl_x64 vars so it works properly with a wrapper prepended.

R=scottmg@chromium.org

Review URL: https://codereview.chromium.org/14861016

git-svn-id: http://gyp.googlecode.com/svn/trunk@1635 78cadc50-ecff-11dd-a971-7dbc132099af