Conform to changes in libhwloc's API (Bug #289)

The 'cpuset' functions in libhwloc have been made more generic and renamed
to 'bitmap' functions.  This patch updates the Mercury runtime system
appropriately.

runtime/mercury_context.c:
    As above.

configure.ac:
    We now require at least version 1.1 of libhwloc (previously we required
    at least 1.0).