gtk-xfce-engine: update to 3.2.0

announcement for 3.2.0 [1] notes that gtk3 engine is not used in
Xfce 4.12 and is disabled by default. For users still wanting gtk3
part build, a PACKAGECONFIG was introduced.

[1] https://mail.xfce.org/pipermail/xfce/2015-February/034198.html



Signed-off-by: Andreas Müller <schnitzeltony@googlemail.com>