meson: generalize libclc usage

So that it's not tied directly to clover.

Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/7034>
diff --git a/meson.build b/meson.build
index bd6a1fc..5958231 100644
--- a/meson.build
+++ b/meson.build
@@ -72,6 +72,7 @@
     'xvmc',
   ]
 endif
+with_clc = false
 
 with_intel_tools = with_tools.contains('intel') or with_tools.contains('intel-ui')
 with_imgui = with_intel_tools or with_vulkan_overlay_layer
@@ -802,15 +803,19 @@
     error('OpenCL Clover implementation requires at least one gallium driver.')
   endif
 
-  dep_clc = dependency('libclc')
+  with_clc = true
   with_gallium_opencl = true
   with_opencl_icd = _opencl == 'icd'
 else
-  dep_clc = null_dep
   with_gallium_opencl = false
   with_opencl_icd = false
 endif
 
+dep_clc = null_dep
+if with_clc
+  dep_clc = dependency('libclc')
+endif
+
 gl_pkgconfig_c_flags = []
 if with_platform_x11
   if with_any_vk or with_egl or (with_glx == 'dri' and with_dri_platform == 'drm')