third-party/hwloc/Makefile: enable usage of qthreads in OS X

OS X uses BSD touch, which doesn't define the -d option. Instead, there's the -A option that does the same thing (but with different values).

With this change, chapel works on OS X with CHPL_TASK=qthreads.

I'm afraid of some kind of Linux that doesn't report Linux to ``uname -s``, so I was wondering whether we should do a feature check in the touch command (try the Linux flag, then fallback to the BSD flag), but I decided to submit the pull request and see what you guys think about it.