Fix incorrect Fortran preprocessing candidates.

* This partially reverts "added support for C89/90 preprocessor" which only
  covered up an already bad situation.

Using the C preprocessor on Fortran sources does ultimately not work
since it doesn't know about Fortran-specific features like the _OPENMP
macro and is chronically likely to pick up the cpp belonging to
another compiler (e.g. to system gcc when compiling with xlf on Linux/ppc).