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.

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

Configure gets triggerd by CDO's configure call for non installing libcdi.la
through a shell variable, so '--enable-cdi-lib' is removed (refs #492)

This is the default build for CDO. 'make distcheck' still checks 
building + installing the library + testing app (refs #492)

Added x86_64 support in cfortran.h (automatic build of fortran bindings)
Print out configuration settings

refs #82