Fixes LLVM library detection.

I must have had a dirty cmake configuration which didn't encounter this issue.
Make sure we are using LLVM_DEFINITIONS to know where we are linking to libraries at.
Link against libLLVM-${VERSION}, the other one was linking us against static libraries...
Merge pull request #1412 from Sonicadvance1/fix-LLVM-detection

Fixes LLVM library detection.