This is helpful on systems with multiple LLVM installations.

For example, OpenBSD ships with LLVM in the base system, but without
support for extra targets.

A full LLVM version can be installed using the ports system, but even
when it is, `cmake` is not going to pick it up unless `/usr/local` is
explicitly added to the `cmake` search paths.

Having the full `llvm-config` path printed on error is helpful to
understand that the detected LLVM version was not the expected one.