CONTRIB: Build FMU on Mac OS 64 bit (darwin64)

Before this commit, contrib/fmu/Makefile assumed linux64, and ld flags
were not appropriate for Mac OS.  As of this commit, building of the
example (via robotfmu.py) FMU for darwin64 succeeds for
Mac OS X version 10.6.8 with GCC 4.2.1 (Apple Inc. build 5666),
and model.xml, which uses the example FMU, runs in vergil of Ptolemy II
at revision 69691.