Everything was always compiled without MPI

A small missing '$' caused a lot of mischief. Augmented also the tests
to detect something like this in the future

configure.ac

@@ -70,7 +70,7 @@ fi
 
 dnl mpi
 AC_ARG_WITH(mpi, [AS_HELP_STRING([--with-mpi=[[yes|no]]], [compile with MPI. Default: yes])],,[with_mpi=yes])
-AM_CONDITIONAL([WITH_MPI],[test x"with_mpi" = x"yes"])
+AM_CONDITIONAL([WITH_MPI],[test x"$with_mpi" = x"yes"])
 if test x"${with_mpi}" = x"yes"; then
        AC_DEFINE([WITH_MPI], [1], [use MPI])
 fi

test/shared_sources/setup_mpi.F90

@@ -81,6 +81,12 @@ module mod_setup_mpi
       call mpi_comm_rank(mpi_comm_world,myid,mpierr)
       call mpi_comm_size(mpi_comm_world,nprocs,mpierr)
 
+#ifdef WITH_MPI
+      if (nprocs <= 1) then
+        error stop "The test programs must be run with more than 1 task to ensure that usage with MPI is actually tested"
+      endif
+#endif
+
     end subroutine