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.