I could not reproduce the Pango issue. It has always built fine for me. Today I tried building from a clean install of MXE and again it worked. I can't debug an issue that I can't reproduce.
Since you now managed to build gtk3 and gtkmm3, you must have built pango as well, so I suppose your failed attempts had been caused by the missing dependency. That's unrelated to the reported issue, which is a linker error on some missing symbols from libfontconfig. Have you actually reproduced the linker error? Has anyone else? If not, I suspect the problem may be caused by a broken build of libfontconfig on the reporter's system. My first suggestion then would be to ask the reporter to restart from a clean install and try again.
Regarding your suggestion to build native gdk-pixbuf, I don't think we should do that. In order to build it we must first build all its dependencies (and instruct the dependent packages to find them, as they would be installed in a nonstandard place). So that means patching many packages, with patches that we'd have to maintain forever, and that haven't any actual purpose except to avoid installing gdk-pixbuf, which many people will have installed already. It's so much cleaner and easier to just add the needed requirement. I understand the desire to keep the list of requirements to a minimum, but if a package is indeed required, it is a requirement. There are already other requirements that are only needed by specific sets of packages, for example I don't think that every package needs python, ruby and cmake. If you wish, you may add a note to the list of requirements that gdk-pixbuf is only needed if you want to build gtk packages.