Workaround the CMake dependency lookup returning a different LLVM to llvm-config.

Bug: https://bugs.gentoo.org/907965
--- a/meson.build
+++ b/meson.build
@@ -1795,6 +1795,7 @@ if _llvm.allowed()
     static : not _shared_llvm,
     fallback : ['llvm', 'dep_llvm'],
     include_type : 'system',
+    method: 'config-tool',
   )
   with_llvm = dep_llvm.found()
 endif