...but disable them for now.  Configs will be added for the projects to
support Vulkan-enabled builds which will then enable the builds of these
sources.  Extra commands and items will be needed for the GSK resources
along with ensuring GSK_RENDERER_GSK being defined for the build of GDK,
GDK-Win32 and GSK so that the builds of Vulkan-enabled builds can be done
properly.

Filter out the Vulkan sources from the 'dist hook' rules in
gsk/Makefile.am as we don't want to in turn include them twice in the
projects when the 'make dist' is performed on a system with Vulkan
builds enabled.