Use pkg-config for builing/linking against hdf5

The next releases of Debian and Ubuntu have moved where the libraries and headers are located. This means that h5py won't be able to be built against them. This adds support for using pkg-config (via the pkgconfig python module) to find the headers and libraries. To avoid requiring installation of pkgconfig before installing h5py, this includes support for setup-requires in the setup.cfg file.