-Add configuration in Clutter projects to add option to build Clutter with
the GDK3 backend in addition to the Win32 backend
-Add another preconfigured clutter-config.h.win32_GDK which contains
backend configs for both GDK3 and Win32 windowing and input.
Update output DLL/LIB names to be consistent with the autotools output,
due to the newl-introduced multi-backend feature (only the Win32 backend
is supported here for now, GDK3 support will be added when it stabilizes)
