The idea is that when an additional feature is enabled, we should run the same
tests as without it plus additional tests for the feature.

This is the first step towards building serial versions of tests also when MPI is enabled.

This is the first step towards switching to the parallel test harness.

The configure options enabling the interfaces are named consistently with the
options enabling Fortran interfaces.

1. Check for FC only if one of the Fortran interfaces is requested.

2. Fail if the Fortran ISO C interface is enabled but FC is not found.

3. Check for F77 only if the C Fortran interface is requested and either FC was
   not found or the user set F77 explicitly. In the latter case, we make sure
   that the generated library is compatible with both FC and F77.

4. Check for Fortran interfaces of MPI and YAXT only when needed and do not fail
   if either of them not found: we can still provide the C Fortran interface.

5. Refactor the Automake conditionals: remove redundant, rename the required in
   a consistent way. Conditionals from acx_options.m4 left unchanged since they
   might affect the integration into CDO.

6. General clean-up and refactoring.

7. Temporarily drop support for the experimental ruby and python interfaces.

Temporarily set USE_MPI macro in the makefile to guarantee it is accounted for in all relevant cases.

Probably this will need to be revisited.

1. Remove 'cdilib.c' when 'make clean'.
2. Remove the Fortran module when 'make mostlyclean'.