When LLVM is built inside of a git repo (even way below, e.g. /usr/ports/.git
exists, and LLVM is built in /usr/ports/devel/llvm50/work), its version
becomes something like 5.0.0git-f8ab206b2176.

New meson versions already handle this, but we support older versions too.

Fixes: 673dda83

 ("meson: build "radv" vulkan driver for radeon hardware")
Reviewed-by: Dylan Baker <dylan@pnwbakers.com>