configure.ac: introduce --{disable,enable}-lib-lttng-ctl

configure.ac: introduce --{disable,enable}-lib-lttng-ctl

The goal is to be able to only build liblttng-ctl, for example without
needing to build bin/lttng.

Since liblttng-ctl is required when building some of the binaries,
./configure will fail if --disabled (explicitly) unless those binaries
are --disabled too.

Previously, the following would result in liblttng-ctl not getting
built, but it now gets built by default:

  ./configure \
    --disable-bin-lttng \
    --disable-bin-lttng-relayd \
    --disable-bin-lttng-sessiond

Change-Id: I9338c46e64c031360aa762a3ce891511a3dbba39
Signed-off-by: Christophe Bedard <christophe.bedard@apex.ai>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>