intel: Allow using intel_clc from the system

With -Dintel-clc=system, the build system will search for an `intel_clc`
binary and use it instead of building `intel_clc` itself.

This allows Intel Vulkan ray tracing support to be built when cross
compiling without terrible hacks (that would otherwise be necessary due
to `intel_clc`'s dependence on SPIRV-LLVM-Translator, libclc, clang, and
LLVM).

Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/24983>

meson_options.txt

@@ -616,9 +616,12 @@ option(
 
 option(
   'intel-clc',
-  type : 'feature',
+  type : 'combo',
   deprecated: {'true': 'enabled', 'false': 'disabled'},
   value : 'disabled',
+  choices : [
+    'enabled', 'disabled', 'system',
+  ],
   description : 'Build the intel-clc compiler (enables Vulkan Intel ' +
                 'Ray Tracing on supported hardware).'
 )