Rebased. I also noticed this woefully out of date line:
- PGAC_PATH_PROGS(LLVM_CONFIG, llvm-config llvm-config-7
llvm-config-6.0 llvm-config-5.0 llvm-config-4.0 llvm-config-3.9)
+ PGAC_PATH_PROGS(LLVM_CONFIG, llvm-config llvm-config-17
llvm-config-16 llvm-config-15 llvm-config-14)