meson: allow building with vulkan beta extensions enabled.

This is just a precursor to anyone enabling beta stuff later.

Reviewed-by: Emma Anholt <emma@anholt.net>
Reviewed-by: Eric Engestrom <eric@engestrom.ch>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/13685>

meson.build

@@ -333,6 +333,11 @@ else
   with_dri_platform = 'none'
 endif
 
+with_vulkan_beta = get_option('vulkan-beta')
+if with_vulkan_beta
+  pre_args += '-DVK_ENABLE_BETA_EXTENSIONS'
+endif
+
 _platforms = get_option('platforms')
 if _platforms.contains('auto')
   if system_has_kms_drm