Need to add OpenMP flag.

[patrick-kidger]

setup.py doesn't currently enable OpenMP for the C++ extensions. (Which by-the-by we may wish to think about the future of as maintaining those is going to be rather a hassle.)

See e.g.
https://github.com/patrick-kidger/signatory/blob/master/setup.py#L33
for an example of how to enable the OpenMP flags.

(Whilst we're at it we should also add the fvisibility flag; see the same example.)

[patrick-kidger]

Closing as we don't currently use C++.