`emerge --info gtk+` reports that GTK is built with flags USE="aqua (prefix) (-cups) -debug -doc (-introspection) -jpeg -jpeg2k -test -tiff -vim-syntax -xinerama" dev-java/gnu-classpath requires this header for GTK peer support compilation
for that you need -aqua. aqua enables the native OSX port of GTK+
Can X11 and Aqua versions of GTK coexist in one prefix?
I don't think so. One would have to slot them, but I don't think this will work properly at all. I agree, though, that in this case (and some others) you just would like to have the X11 port available too. Maybe the build system of GTK can do multiple targets these days
right, xfce-base/libxfce4ui-4.8.0 runs into this as well. are there any particular drawbacks for just running with the X11 backend?
gtk+-3 can do it, but it just takes someone to experiment with it
gtk+-3.0.9 fails compile for me, see bug 366897