meson: Make shader-cache a trillean instead of boolean

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>