Added support to setup to try version specific llvm-configs on linux

Hallo llvmpy-devs and thanks for an awesome project.

I recently tried to install llvmpy from pypi on Ubuntu 14.04 LTS. The install fails (I think) because I have several versions of llvm installed and therefore llvm-config is a symlink to the config of the newest version, which in my case is 3.4.

This PR attempts to fix that, by attempting to use the version specific llvm-configs (e.g. `llvm-config-3.3`) if the version returned by the generic one is insufficient and we are on a Linux system. I don't rightfully know, if this could be extended to other operating systems as well. Probably, maybe, this is also an issue on Mac and might be fixed in the same way, but that will need to be tested.

Admittedly, it is sort of a hack, so any suggestions for a cleaner way to do it, would be most welcome.