meson: Fix automagic build of msdk plugin

When building the msdk plugin even if libmfx is found, unless the
plugin is explicitly enabled we should not error out if msdk
dependencies are not found.

Also give an error message when we don't build the plugin on Windows
because we're not building with MSVC.
8 jobs for fix-msdk-auto in 18 minutes and 25 seconds (queued for 3 seconds)