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.build

@@ -252,12 +252,11 @@ with_any_broadcom = [
 ].contains(true)
 
 if ['x86_64'].contains(host_machine.cpu_family())
-  with_intel_clc = get_option('intel-clc').enabled()
-  with_intel_vk_rt = with_intel_vk and with_intel_clc
+  with_intel_clc = get_option('intel-clc') == 'enabled'
 else
   with_intel_clc = false
-  with_intel_vk_rt = false
 endif
+with_intel_vk_rt = with_intel_vk and get_option('intel-clc') != 'disabled'
 
 with_any_intel = [
   with_gallium_crocus,