cdi 1.8.x zaxis tropopause backport

See merge request !1

* fflush only moves data to the page cache, but does not ensure the data
  is actually written into permanent storage.

Fail the configure script if ISO interface is requested but Fortran compiler is not found or disabled.

Uwe Schulzweida authored 5 years ago and Sergey Kosukhin committed 5 years ago

Fix the problem of grid comparison occuring when input files on the same grid have different GRIB key numberOfGridInReference.

The previous code resulted in the following error when compiled with
GCC 6.4.0 and FCFLAGS=-std=-f2008:

TS 29113/TS 18508: Noninteroperable array FPTR at (1) to C_F_POINTER: Type shall have a character length of 1

Moreover, the code did not really reflect how the string manipulation
was implemented in src/mo_cdi.f90. The updated version is supposed to
be closer to what is required in the real code.

Fail the configure script if ISO interface is requested but the compiler does not support CHARACTER interoperability.