clang plugin: allow overriding llvm-config binary

ppa:h-rayflood/llvm has a llvm-3.2 (and clang) with the llvm-config binary named llvm-config-3.2 instead (that is, version suffixed); this allows building the clang plugin with `make LLVM_CONFIG=llvm-config-3.2` to succeed (since the package doesn't use update-alternatives for some reason).

This does not change the default behaviour.