Cleanup: lttng-ust library detection

Simplify lttng-ust detection code.

Also remove the --with-lttng-ust-prefix configure option since we don't
offer it for other libs and it's based on user variables which the build
system shouldn't be messing with.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>