I'm running up against an issue building on Mac with an X11 (non-Quartz) GTK. It appears that gui/Makefile is only building the osx_open. files when mac_native is enabled (line 251). However the relevant symbols are called (e.g. in gui/helpfiles.c) whenever OS_OSX is defined (line 2559). So it appears there is some mismatch in the logic. I believe osx_open. should be always compiled on Mac, regardless of whether the Quartz or X11 GTK is being used.
For some years now we've been assuming that X11-based GTK on macOS is over and done with, but if that's not the case we need to rethink. No doubt you're right about the inconsistency; we'll look into the issue. But it would help to motivate us if you could you give us an idea of why X11 is still of interest.
GTK/X11 is a supported configuration in MacPorts, and not uncommon on legacy systems. If you'd like to drop support, that is completely fine, and we can indicate as much in the MacPorts package. But it would help to throw an error during configuration rather than at build-time. On the other hand if it's a one-line fix then that may be just as easy.
OK, I've made a change to gui/Makefile.in in git master, which I think should fix this issue.
Is this issue settled, or are there any leftovers?
It's settled.