Fix external C API for astropy.wcs

This does two things:

It fixes the external C API code (which was just plain broken before, as it never got fully converted over from pywcs).

It installs the wcslib header files alongside the astropy.wcs header files so that *if* the external library wants to use the wcslib we ship with astropy, rather than the system one, they can.  (Though that decision is entirely up to the third-party library -- this PR doesn't obligate them to use it).

All of this is required so work we are doing locally to have `drizzlepac` depend on astropy, rather than pywcs.