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.
Fix shared libraries on Darwin platform.

Mac OS X doesn't have the precise equivalent of /etc/ld.so.cache where shared libraries are automatically located by name. The closest available functionality is to tag each shared library with the install_name of i.e. @rpath/libomu.so, where @rpath is expanded, during linking, to a list of paths constructed throughout the associated dependency chain. Essentially, it lets us piggy-back onto the -Wl,-rpath,... that's already being done for other platform.

This patch uses the install_name_tool binary to retroactively change the install_name. The alternative is to supply it during linking, as -Wl,-install_name,... but it's not obvious to me how that solution could be elegantly patched into the existing Makefiles without breaking other platforms.
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.
Tone down some over-ambitious warnings.

The comment warnings should possibly be heeded and changes made to the source instead. The unused parameter warning I'm not sure I philosophically agree with.
Be more explicit about order of logical operations.

(Motivated by GCC warning)