gtk2-engines: bump, rename config option and convert to AUTOTARGETS

gtk2-engines is bumped from 2.9.1 to 2.20.2 (the latest available
version compatible with Gtk2). The package is converted to the
AUTOTARGETS infrastructure.

The config option is renamed so that it matches the package name
(gtk2-engines instead of libgtk2-engines).

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>