meson: add feature option for use of system Clang headers at runtime

Enabling or disabling the "microsoft-clc" option previously changed
shared logic for all compiler/clc users, which was surprising.

In addition, the option to avoid the use of system Clang headers at
runtime is useful outside the scope of Windows.

Separating the two concepts by making this a neutral feature option
addresses both matters.

Signed-off-by: Dor Askayo <dor.askayo@gmail.com>
Reviewed-by: Jesse Natalie <jenatali@microsoft.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/23255>

meson_options.txt

@@ -183,6 +183,13 @@ option(
   description : 'Link libclc SPIR-V statically.',
 )
 
+option(
+  'opencl-external-clang-headers',
+  type : 'feature',
+  description : 'Use system Clang headers at runtime for OpenCL kernel ' +
+                'compilation.',
+)
+
 option(
   'd3d-drivers-path',
   type : 'string',