The upcoming GCC 9 release will contain the first version of gfortran that provides the Fortran 2018 ISO_Fortran_binding.h header file, which MPICH requires in order to build MPI 3's mpi_f08 module.  If anyone is interested in testing, please checkout the current GCC trunk and submit any related issues to the gfortran developers at fortran@gcc.gnu.org.  Alternatively, if its easy to explain how to run any related tests in the MPICH test suite, please let me know.  So far, I have verified that mpi_f08.mod gets installed when MPICH is built with a recent checkout of the gfortran trunk.