Clang (like LLVM), very annoyingly refuses to provide pkg-config, and
only provides cmake (unlike LLVM which at least provides llvm-config,
even if llvm-config is terrible). Meson has gained the ability to use
cmake to find dependencies, and can successfully find Clang. This change
attempts to use cmake to find clang instead of a bunch of library
searches, when paired with -Dcmake_prefix_path we can much more reliably
use cmake to control which clang we're getting. This is only enabled for
meson >= 0.51, which adds the required options.

Reviewed-by: Eric Engestrom <eric.engestrom@intel.com>