Make the namespace cleanliness check work on Mac OS X.

Fix an old problem: the check_objs target would fail on Mac OS X because
exported symbols on that platform are prefixed with "_".

Mmmake.common.in:
    Allow object symbols to have an optional system-specific prefix.

    Delete a workaround for a problem that was fixed five years ago.

configure.ac:
    On Darwin systems set the appropriate prefix.