Fix bug and ambiguity when using LLVM_SYS_<VERSION>_PREFIX
Before this change there was a bug where if you used LLVM_SYS__PREFIX, build.rs would only look for llvm-config inside the prefix, and not the other binary candidates llvm-config-X, llvm-config-X.Y and llvm-configXY.
Additionally with this change, if a prefix is specified, build.rs will look for llvm-config ONLY inside the prefix. The assumption being that the user know what they're doing, and may be trying to link to a specific build or fork of LLVM.
If not prefix is set, $PATH is searched.