Remove unused import
Make pkg-config name for healpix_cxx lower case
Add Cython-generated .cpp files to MANIFEST.in

This has the effect of including them in tarballs generated by
'python setup.py sdist'.
Add library paths to runtime_library_dirs

This ensures that shared libraries are in the rpath, so that they do not
have to reside in the default shared linker path.
Add external library build arguments to individual extensions

...rather than to the build_ext instance.