librdmacm: Add a check for libpthread during librdmacm configure.

librdmacm: Add a check for libpthread during librdmacm configure.

Add a check for libpthread during librdmacm configure.  This will add
libpthread to the list of libraries that librdmacm is linked to.
Currently librdmacm gets libpthread implicitly through libibverbs, but
this breaks when using a linker that does not implicitly link with such
dependencies; eg the new gold linker is such a linker:

<http://wiki.debian.org/qa.debian.org/FTBFS#A2009-11-
02Packagesfailingbecausebinutils-gold.2BAC8-indirectlinking>

Addresses: http://bugs.debian.org/555380
Addresses: https://bugs.launchpad.net/ubuntu/+source/librdmacm/+bug/687983

Signed-off-by: Roland Dreier <rolandd@cisco.com>
Signed-off-by: Sean Hefty <sean.hefty@intel.com>