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.build

@@ -775,9 +775,19 @@ if get_option('buildtype') == 'debug'
   pre_args += '-DDEBUG'
 endif
 
-if get_option('shader-cache')
-  pre_args += '-DENABLE_SHADER_CACHE'
-elif with_amd_vk
+with_shader_cache = false
+_shader_cache = get_option('shader-cache')
+if _shader_cache != 'false'
+  if host_machine.system() == 'windows'
+    if _shader_cache == 'true'
+      error('Shader Cache does not currently work on Windows')
+    endif
+  else
+    pre_args += '-DENABLE_SHADER_CACHE'
+    with_shader_cache = true
+  endif
+endif
+if with_amd_vk and not with_shader_cache
   error('Radv requires shader cache support')
 endif
 
@@ -1108,7 +1118,7 @@ if cc.has_function('dl_iterate_phdr')
   pre_args += '-DHAVE_DL_ITERATE_PHDR'
 elif with_intel_vk
   error('Intel "Anvil" Vulkan driver requires the dl_iterate_phdr function')
-elif with_dri_i965 and get_option('shader-cache')
+elif with_dri_i965 and with_shader_cache
   error('Intel i965 GL driver requires dl_iterate_phdr when built with shader caching.')
 endif
 

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(