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'.

This is supposed to be mainly reformatting with a couple of refactoring
touches. No functional changes are intended.

Otherwise, the debugging becomes rather annoying.

This makes the test more transparent.

This reverts commit 584cabb1.

* We do this at least temporarily to monitor that the test works.

Unify PPM CORE location interface and check whether the library is really available in the configure script.