The tests and microbenchmarks are re-compiled if the GPI-2
libraries have changed.

This allows for `make distclean` (subdirs) in old automake versions.

However, they are still commented (not compiled).

Allows for make uninstall.

Set automatically to MPI if CC=mpi*

The user can specify or not the path to OFED.

There is no need for copy and/or use the external macro
for the module flag.