GTK3 branch builds GTK2 by default

[diabl0w]

I don't think the GTK3 branch builds GTK3 by default via cmake, instead I get a GTK2 binary.
There is some logic for doing so, but I don't believe that option gets properly set anywhere to GTK3.

[diabl0w]

calling cmake -GNinja -DBUILD_USING_GTK3=ON allows it to build fine with cmake, but this should be the default for this branch without requiring it to be specified

[adriweb]

Yes, once the GTK3 support is considered stable and ready it'll become the default (and the branch merged anyway)