[GTK] [CMake] Build the plugin process against GTK+ 2

https://bugs.webkit.org/show_bug.cgi?id=116374

Reviewed by Gustavo Noronha Silva.

.:

* Source/cmake/FindGDK2.cmake: Added.
* Source/cmake/FindGTK2.cmake: Added.
* Source/cmake/OptionsGTK.cmake: Look for GTK2 and GDK2.
* Source/cmake/WebKitMacros.cmake: Abstract WebKit2 IPC generation here so it
can be shared between the WebKit2 library and the plugin process.

Source/WebCore:

* PlatformGTK.cmake: Split off the GTK+-dependent sources into
libWebCorePlatformGTK and compile libWebCorePlatformGTK2 when
WebKit2 is enabled.

Source/WebKit2:

* CMakeLists.txt: Use the new GENERATE_WEBKIT2_MESSAGE_SOURCES macro so we
can share messaging file generation with the plugin process.
* PlatformGTK.cmake: Add more files to the plugin process source list. Properly
generate message sources, add the appropriate include paths, defines, and libraries
for the plugin process.

Tools:

* TestWebKitAPI/PlatformGTK.cmake: Add WebCorePlatformGTK to the list of
libraries required for WebCore unit tests.

git-svn-id: http://svn.webkit.org/repository/webkit/trunk@160728 268f45cc-cd09-0410-ab3c-d52691b4dbfc