Discovering GTK+/OSX with autoconf

I'm looking for an elegant way to discover the frameworks using autoconf. So far I just have the inelegant way, of testing for the existence of the framework directories. The frameworks are not installed with pkg-config, so I can't discover them the usual way. Is there a recommended best practice?

pkg-config works

pkg-config works fine...

export PKG_CONFIG_PATH=/Library/Frameworks/Gtk.framework/Resources/dev/lib/pkgconfig:\
/Library/Frameworks/Cairo.framework/Resources/dev/lib/pkgconfig:\
/Library/Frameworks/GLib.framework/Resources/dev/lib/pkgconfig

Easy.