meson: use a feature option for shader_cache

Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Acked-by: Alyssa Rosenzweig <alyssa.rosenzweig@collabora.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/20409>

meson_options.txt

@@ -200,9 +200,7 @@ option(
 )
 option(
   'shader-cache',
-  type : 'combo',
-  value : 'auto',
-  choices : ['auto', 'true', 'false', 'enabled', 'disabled'],
+  type : 'feature',
   description : 'Build with on-disk shader cache support.',
 )
 option(