gtk/meson.build: Fix linking on Visual Studio 2013

Hi,

From the commit message (in italics):

Visual Studio 2013's linker does not suport /WHOLEARCHIVE:, so link_whole: needs to be updated in order to fix this (if that is possible, since we can't know what static libraries have been linked_whole'ed in a static library target). In the meanwhile, just explicitly extract the objects from the static libraries that will form the final GTK DLL., since it is quite clear what static libraries we are really linking into the final GTK DLL.

Unfortunately, we can't try to fix Meson for /WHOLEARCHIVE: on pre-Visual Studio 2015 Update 2 as we can only know the objects that is built into the static .lib after it has been built by running lib /list /nologo <static_lib> against that built static .lib.

For the static lib's that have the link_whole:... stuff in their target definition, we can just leave those alone since it doesn't really matter here.

With blessings, thank you!