[GTK] Expose more of the configuration step to the gyp build
https://bugs.webkit.org/show_bug.cgi?id=111213

Patch by Martin Robinson <mrobinson@igalia.com> on 2013-03-01
Reviewed by Dirk Pranke.

Expose more of the configuration phase to the gyp build, including WebKit features
and binary paths.

* gyp/Configuration.gypi.in: Add templatized gyp variables for the new configuration bits.
* gyp/autogen.sh: Properly link up the WebKit features script. So we can access it
relatively as if we were at the top-level of the source directory.
* gyp/configure.ac: Export the WebKit features.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@144506 268f45cc-cd09-0410-ab3c-d52691b4dbfc