improve treatment of zero-order hold for model inputs and outputs
Add a flag for Darwin linking behaviour.

Darwin does want to be able to resolve all symbols during linking, but there's no need to bake everything into libomu.so, as we do in for windows. Instead, we can supply -lhqp during both libomu.so linking and executable linking. We overload the hqp_libs_resolved flag with the new content 'partial' to flag this behaviour.
Use the right library path on 64-bit platforms.

More recent ADOL-C versions install into ./lib64 on 64-bit platforms. Add a check in our configure.in script to default to lib64 when it exists.
add initial support for Mac OSX
add install_name to shared libraries for Darwin platform