buildroot

pcmanfm: add optional support for gtk3

pcmanfm can now use gtk3+ so use that over gtk2+ if it's available.
Since it's only x11-safe for the moment the comment and deps other than
gtk stay the same.

Signed-off-by: Gustavo Zacarias <gustavo@zacarias.com.ar>
Signed-off-by: Thomas Petazzoni <thomas.petazzoni@free-electrons.com>