fix: Use python-config 'ldflags' instead of 'libs'

fix: Use python-config 'ldflags' instead of 'libs'

This allows building the python plugin provider against a Python
installation that is not in the default system path.

The output of '--ldflags' is the same as '--libs' with the added linker
path to the python library.

Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Change-Id: Idf0171bdc181a7d6d7593021dab957437772b2f5
Reviewed-on: https://review.lttng.org/c/babeltrace/+/1558
Tested-by: jenkins <jenkins@lttng.org>
Reviewed-by: Philippe Proulx <eeppeliteloop@gmail.com>