* And on AIX it isn't.

This reverts commit fe250209a1d87d4a6e5378acf0bb98db2bfdb63c.

* The previous commit neglected special cases for the CDI-pio variant.

* pthread_once_t provides the same semantics.

* It depends on having a Fortran compiler in the first place.

* This fix is temporary in nature since the underlying problem, too tight
  integration of file I/O layer and stream abstraction, is even
  aggravated with this patch.

* Otherwise netcdf.h will define some MPI declarations.