LDADD PTHREAD_LIBS to fix librados-config linking

librados-config depends on libglobal which in turn
reference symbols in pthread; this caused issues
when linking due to the indirect dependency via
libglobal.

Signed-off-by: James Page <james.page@ubuntu.com>
Merge pull request #54 from javacruft/wip-pthread-linking

LDADD PTHREAD_LIBS to fix librados-config linking

Reviewed-by: Sage Weil <sage@inktank.com>