Update WebKitDirs.pm for new Windows paths
https://bugs.webkit.org/show_bug.cgi?id=107714

Reviewed by Daniel Bates.

* Scripts/webkitdirs.pm: For each existing Windows environment
variable, also include creation of the 'new' variables. The
'old' variables will be removed in a future update.
(windowsSourceSourceDir): New helper routine to return the
actual 'Source' folder of the WebKit source tree.


git-svn-id: http://svn.webkit.org/repository/webkit/trunk@142692 268f45cc-cd09-0410-ab3c-d52691b4dbfc