[GTK] [CMake] Add a production build type

[GTK] [CMake] Add a production build type
https://bugs.webkit.org/show_bug.cgi?id=126179

Reviewed by Daniel Bates.

* Source/PlatformGTK.cmake: When the API tests are enabled compile the WebKit GObject
API unit tests. The inclusion of these build files was inadvertently removed in an earlier
patch.
* Source/cmake/OptionsGTK.cmake: When in production mode turn off all tools and tests and
use the autotools linker script.

git-svn-id: https://svn.webkit.org/repository/webkit/trunk@161093 268f45cc-cd09-0410-ab3c-d52691b4dbfc