This way, we avoid target-specific flags '-DUSE_MPI', which result into
target-specific rules that do not use Libtool for the compilation.

We need all this because the configure script accepts only absolute paths for MPI_LAUNCH.

This allows for sanitization of the build environment on the command line.

For example:
  export BUILD_CC='bad stuff'
  ./configure 'BUILD_CC='
  make

Remove declaration of the 'BUILD_C_INCLUDE' and 'BUILD_FC_INCLUDE' precious variables from the configure script since they are not used.

The shortcut for the ISO C99 test, which simplifies the implementation, is copied from PPM.

Unify PPM CORE location interface and check whether the library is really available in the configure script.

Unify YAXT location interface and check whether the library is really available in the configure script.

See https://github.com/Unidata/netcdf-c/blob/cd39b9d2d83fed40a45e8a8dd748a6bceae5ddfa/include/netcdf_par.h#L13-L18

Also, get rid of ac_fn_c_check_header_mongrel in the configure script.