add --disable-dlopen, use correct configure flag for MPI threading support

* use --disable-dlopen to avoid problems like with Rmpi:
```
mca: base: component_find: unable to open .../lib/openmpi/mca_paffinity_hwloc: .../lib/openmpi/mca_paffinity_hwloc.so:
undefined symbol: opal_hwloc_topology (igno     red)
 mca: base: component_find: unable to open .../lib/openmpi/mca_carto_auto_detect:
.../lib/openmpi/mca_carto_auto_detect.so: undefined symbol: opal_carto_base_graph     _get_host_graph_fn (ignored)
```
* use correct configure flag to enable MPI threading support (`--enable-mpi-thread-multiple` instead of `--enable-mpi-threads`)