build: Backport addition of librt check to configure.ac

build:  Backport addition of librt check to configure.ac

A while back, a master-only change was made to check for librt which
should probably have been cherry-picked to 13 at that time.  Sometime
between then and now, part of that change did make it into 13 but it
was incomplete and non-functional.  This patch backports the rest
of the librt check and allows the link of libasteriskpj to use the
results.

Change-Id: I1424008fd8c90f389dda53162ec4a340b253a3c1