meson/gallium: Add an option to not use LLVM for gallium draw module

We'd like to use one Mesa build environment which builds our CL compiler
stack (which needs Clang/LLVM) and which builds our GL driver. The GL
driver doesn't really need LLVM support, and since we're statically
linking LLVM, removing it from the driver drastically reduces our DLL
size on disk.

Acked-by: Eric Anholt <eric@anholt.net>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/9259>

meson_options.txt

@@ -321,6 +321,12 @@ option(
   choices : ['auto', 'true', 'false', 'enabled', 'disabled'],
   description : 'Whether to link LLVM shared or statically.'
 )
+option(
+  'draw-use-llvm',
+  type : 'boolean',
+  value : 'true',
+  description : 'Whether to use LLVM for the Gallium draw module, if LLVM is included.'
+)
 option(
   'valgrind',
   type : 'combo',