Consider introducing AIX support

[Ian Culverwell]

ROPP-10 beta reviewers Christian Marquardt and Leonid Butenko say:

Several changes (mostly to the automake build system) have been introduced for
making it possible to compile and run ROPP v10 on AIX; most of them are related
to the native AIX linker not being as clever as linkers on modern Linux systems.
They therefore require correctly ordered lists of all libraries to be linked.
Some modifications to a small number of Fortran files were also required to make
them conforming with the Fortran 9x standard; GNU compilers tend to be more
relaxed about them.

The patches we required to build ROPP on AIX using the IBM's native
compiler suite are available in the following merge request:
https://gitlab.eumetsat.int/ro/ropp/merge_requests/6.

Consider, at a low priority, for ROPP-11.0.

[Ian Culverwell]

Unlikely that anyone will ever have the time to do this. Moving to "whenever".