For reference, these are bugs raised against the gtk-osx project related to this issue:
https://bugzilla.gnome.org/show_bug.cgi?id=776216 https://bugzilla.gnome.org/show_bug.cgi?id=776219 https://bugzilla.gnome.org/show_bug.cgi?id=776295
I've updated the instructions and made changes to the build here
https://github.com/geany/geany-osx
so the generated binaries should be usable on any OS X 10.6 or newer. I've also re-uploaded the binary here
http://download.geany.org/geany-1.29_osx.dmg
so everything should work now.