do define the OMPI_LIBMPI_NAME macro via the CPPFLAGS.
The issue occurs when Open MPI is configured with
--enable-opal-btl-usnic-unit-tests
Thanks George Marselis for reporting this issue
Refs. open-mpi/ompi#6441
Signed-off-by: Gilles Gouaillardet <gilles@rist.or.jp>