Make ERFA > 1.4.0 automatically pick up version info

[eteq]

As mentioned in #6239, Erfa v1.4.0 includes liberfa/erfa#42, which exposes the ERFA and SOFA versions in the API.

Unfortunately, the current erfa_generator.py doesn't handle these correctly, for three different reasons reasons:

• Some of the functions return const char* . One would expect const char* functions to return strings at the Python layer. However, the regular SOFA functions return strings via "output arguments" - i.e. char * inputs that get populated inside the function. So the current erfa_generator.py machinery doesn't know what to do with returned const char*'s.
• Some of the functions return int . int return values elsewhere in SOFA are status codes, rather than having actual meaning in and of themselves. So the erfa_generator.py machinery handles those functions assuming they are status codes rather than values to just pass through.
• The macros needed to actual determine the various version numbers are set in config.h in the standard erfa C build. But we don't want that in the python erfa. My temporary solution is to just hand copy the relevant 5 macros from config.h to erfaversion.c... but that's a manual solution and is not really a good idea from a maintenance point of view (it should be possible to just copy erfa over wholesale without modification).

These are all solvable problems, but probably require someone with knowledge of the Python C API... But we really should try to solve them by v3.0.0 so that the manual copying I did in #6239 is no longer required.

[bsipocz]

I suspect this isn't going to happen in the next few hours, so I remove the milestone.

[mhvk]

Now regained relevance since the do-it-by-hand was just duplicated in #9323...