roster_update_presence now requires that
roster_process_pending_presence() has been called. Otherwise presence
are set to pending and not processed yet.
The flag hides errors. But we want unit tests to be correct in order
to reveal errors in the main code.
The patch removes tests_unittests_unittests_CFLAGS which makes autotools
use AM_CFLAGS instead. Therefore, unit tests are built with flags
derived from configure.ac.
