Fix: Missing libs dependencies in configure check for lttng-ust-ctl

Fix: Missing libs dependencies in configure check for lttng-ust-ctl

The lttng-ust-ctl library depends on liburcu-{common,bp,cds}.  The
AC_CHECK_LIBRARY macro can't automatically resolve dependents libraries
(ala libtool), so any additionnals dependencies must be manually
specified.

Also, the AC_CHECK_LIB action-if-found case for the lttng-ust-ctl check
is modified to have a similar behavior as the default action, which is
to define HAVE_LIBxxx and append -lxxx to $LIBS, *except* for the later
step. This is to ensure that any future addition of AC_CHECK_LIB after
the one for lttng-ust-ctl will not need to append the liburcu
dependencies or fail unexpectedly.

Acked-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
Signed-off-by: Christian Babeux <christian.babeux@efficios.com>
Signed-off-by: David Goulet <dgoulet@efficios.com>