Fix regression with pthread_sigmask on FreeBSD.

Fix regression with pthread_sigmask on FreeBSD.

* configure.ac: Configure gnulib at the end, not before running
pkg-config.  This restores the behavior before 2012-06-22, when
higher-resolution time stamps were added, and fixes a bug whereby
LIB_PTHREAD was not used and gnulib's part of 'configure'
therefore incorrectly assumed that pthread_sigmask wasn't working.
Fix the problem with -lrt and clock_gettime a different way.
This should complete the fix for Bug#11884.
(pre_PKG_CONFIG_CFLAGS, pre_PKG_CONFIG_LIBS): New shell vars.