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".