Add support for MPICH 3.x

MPICH has stepped up its versioning scheme, jumping from MPICH2 1.x
to MPICH 3.x, but has stayed compatible to the older MPICH2 versions.
Unfortunately, the macro MPICH2_VERSION used by mpi_config.m4 to detect
the package has been renamed to MPICH_VERSION.

This patch fixes the problem by adding a separate test for MPICH_VERSION
ana making sure that the macro starts with the letter "3".