mpi4py: link to CMAKE_DL_LIBS

Required on one of the dashboards at least.

Change-Id: I96d2ddb5351ef8d5c9d9f3ced52c74cac82d9d4d
Merge topic 'mpi4py-dlopen-link' into master

1493ff0c mpi4py: link to CMAKE_DL_LIBS