mdmon: satisfy glibc tls abi requirements with pthreads

mdmon: satisfy glibc tls abi requirements with pthreads

Setting up a proper tls descriptor is required to conform to the abi
[1].  Until it can be implemented in mdmon use pthreads instead of
clone(2) to let glibc handle the details.  The old behaviour can be had
by un-defining USE_PTHREADS.

Note, the "O2" builds need LDFLAGS now to pick up the '-pthread' option.

[1]: http://people.redhat.com/drepper/tls.pdf

Signed-off-by: Dan Williams <dan.j.williams@intel.com>