net/mpich: Unbreak build with GCC 10

checking whether gfortran10 allows mismatched arguments... no
configure: error: The Fortran compiler gfortran10 will not compile files that call
the same routine with arguments of different types.

http://package18.nyi.freebsd.org/data/121amd64-default-PR244494/2020-06-10_18h04m44s/logs/errors/mpich-3.2.1_6.log

PR: 246700