libmusicbrainz: Explicitly link with libpthread.a

Workaround the issue discussed here:
   http://sourceware.org/bugzilla/show_bug.cgi?id=15126

Signed-off-by: Drew Moseley <drew_moseley@mentor.com>
Merge pull request #27 from drewmoseley/new-toolchain

libmusicbrainz: Explicitly link with libpthread.a