meson: Make shader-cache a trillean instead of boolean

So that it can be implicitly disabled on windows, where it doesn't
compile.

v2: - Use an auto-option rather than automagic.
    - fix shader_cache check (== -> !=)
v4: - Use new with_shader_cache instead of get_option('shader-cache')
      elsewhere in the meson build

Reviewed-by: Eric Anholt <eric@anholt.net>

meson_options.txt

@@ -157,8 +157,9 @@ option(
 )
 option(
   'shader-cache',
-  type : 'boolean',
-  value : true,
+  type : 'combo',
+  value : 'auto',
+  choices : ['auto', 'true', 'false'],
   description : 'Build with on-disk shader cache support'
 )
 option(