Rename PLUGINSDIR to BABELTRACE_PLUGINS_DIR and enable it

The configure script uses the PLUGINSDIR variable to set the
installation directory of the plugins on the system. However this
variable is not used when setting this value in the library, the
default value happens to match.

Fix the build system to honor this variable and rename it to
BABELTRACE_PLUGINS_DIR.

Change-Id: I8575d455017f2964e67fdfab609f5c609434d37b
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1681
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>