makefile adjustments for non standard LLVM path

I modified makefile to handle situations where LLVM is not installed system wide (e.g. only compiled from source). The changes are only inside makefile, and includes a line that is not so general (line 4 of makefile) that specifies my location of llvm-config. When merging, this line can be left as is in the original sources, since it must be changed with real llvm-config path