[GTK] Add support for building WebCore to the cmake build

[GTK] Add support for building WebCore to the cmake build
https://bugs.webkit.org/show_bug.cgi?id=116128

Reviewed by Gustavo Noronha Silva.

.:

* Source/PlatformGTK.cmake: Added.
* Source/cmake/OptionsGTK.cmake: Added more logic and variables to support WebCore and properly
defined some existing variables.

Source/WebCore:

* PlatformGTK.cmake: Added.
* platform/gtk/GtkVersioning.c: Include "config.h" because we removed
the autotoolsconfig.h include from the header.
* platform/gtk/GtkVersioning.h: Remove autotoolsconfig.h include to support for
non-autotools build systems.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@150189 268f45cc-cd09-0410-ab3c-d52691b4dbfc