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)
Status Job ID Name Coverage
  Preparation
passed #85237
gst indent

00:00:48

passed #85236
manifest

00:00:40

 
  Build
passed #85238
build fedora x86_64

00:04:19

 
  Test
passed #85239
check fedora

00:03:35

 
  Full Builds
passed #85240
build android arm64 api28

00:03:43

passed #85242
build cerbero cross win32

00:09:41

passed #85243
build cerbero cross win64

00:07:40

passed #85241
build cerbero fedora x86_64

00:07:56