Seems like Mac OS X don't want to configure the X11 extensions (such as shape/xshm/cursor/xrandr) without first include the <X11/Xlib.h> header for some types ?
With the patch, it works fine on Mac OS X with XFree86 include Xft support for anti-aliased fonts. (i.e. when configured --with-x --with-xft, using Apple's X11.app)
fox-1.6-xlib-include.patch