A fix for OpenMP problems on OS X

Created by: twesterhout

For now, we can borrow FindOpenMP.cmake from PyTorch's repo. They maintain a patched version which seems to work on Travis CI with OS X. If I can lay my hands on a Mac, I'll check whether there's a simpler workaround we can use (see points about AppleClang here https://github.com/pytorch/pytorch/tree/master/cmake/Modules).

@gcarleo @PhilipVinc and other OS X users, please let me know whether this works for you.