[GTK] Forcefully disable gtk-doc and gobject-introspection when crossbuilding

[GTK] Forcefully disable gtk-doc and gobject-introspection when crossbuilding
https://bugs.webkit.org/show_bug.cgi?id=146590

Patch by Emanuele Aina <emanuele.aina@collabora.com> on 2015-07-03
Reviewed by Martin Robinson.

* Source/PlatformGTK.cmake: override the cached variables to
forcefully disable gtk-doc and gobject-introspection when
crosscompiling.
* Source/cmake/OptionsGTK.cmake: avoid running gtk-doc to check the
documentation syntax when cross-building.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@186259 268f45cc-cd09-0410-ab3c-d52691b4dbfc