[GTK] [gyp] Add support for building the platform directory
https://bugs.webkit.org/show_bug.cgi?id=113212

Reviewed by Nico Weber.

* WebCore.gyp/WebCoreGTK.gyp: Add support for building the platform directory including
two libraries, one for platform and one for geometry files. This follows the approach
of Chromium.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@146789 268f45cc-cd09-0410-ab3c-d52691b4dbfc