On Windows, don't try to create relative paths across different drives

This is problematic when the generator output is set to a different drive
from the gyp files and sources.

Landed for borenet@google.com

Review URL: https://codereview.chromium.org/19792014

git-svn-id: http://gyp.googlecode.com/svn/trunk@1683 78cadc50-ecff-11dd-a971-7dbc132099af
Fix typo in r1683.

Review URL: https://codereview.chromium.org/19833003

git-svn-id: http://gyp.googlecode.com/svn/trunk@1684 78cadc50-ecff-11dd-a971-7dbc132099af