Update m4/acx_check_strptr_convert.m4.

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.