meson: va: Skip configuration on non-linux environment

VA plugin is linux-only plugin, so we can skip it earlier.
Note that this plugin is making use of libdrm meson fallback,
which is unusable on the other platforms such as Windows

Part-of: <!1946>

sys/va/meson.build

@@ -21,8 +21,9 @@ va_sources = [
   'gstvavpp.c'
 ]
 
+have_va = false
 va_option = get_option('va')
-if va_option.disabled()
+if va_option.disabled() or host_system != 'linux'
   subdir_done()
 endif
 

tests/examples/va/meson.build

+if not have_va
+  subdir_done()
+endif
+
 gtk_dep = dependency('gtk+-3.0', required : get_option('examples'))
 gtk_x11_dep = dependency('gtk+-x11-3.0', required : get_option('examples'))
 x11_dep = dependency('x11', required : get_option('examples'))