Use llvm-config to get the library directory

[BorisCarvajal]

llvm-config will look for the default LLVM installation, you can pass the --llvm-path to force linking to other version.

The old behavior is still there if llvm-config method fails.

[jacob-carlborg]

I’ve tried to use llvm-config before, it didn’t work that well. I don’t remember the exact details.