openmpi: declare lib dependency on target compiler

[mascguy (Christopher Nielsen)]

While the various openmpi-* subports declare a build dependency on the target compiler, they do not declare a lib dependency for it.

The implications are that:

1. The target compiler will be installed for a source build, only to be uninstalled during a reclaim operation.
2. For a binary install, the target compiler will never be installed. So when a user attempts to use the associated compiler wrappers, they will fail with "The Open MPI wrapper compiler was unable to find the specified compiler."

This doesn't provide a good user experience. Instead, we should add a runtime dependency on the target compiler, to avoid this situation.

Of note, this is already being done for mpich-* subports. But apparently this was missed for openmpi.