Don't emit rerun-if-changed on llvm-config if using system LLVM

Don't emit rerun-if-changed on llvm-config if using system LLVM

The code was broken because it printed "llvm-config" instead of the
absolute path to the llvm-config executable, causing Cargo to always
rebuild librustc_llvm if using system LLVM.

Also, it's not the build system's job to rebuild when a system library
changes, so we simply don't emit "rerun-if-changed" if a path to LLVM
was not explicitly provided.