make: Max recursion level (500) exceeded.: Resource temporarily unavailable

[Don Lewis]

I'm seeing a circular dependency between x11-fonts/fontconfig and
x11/xorg-libraries.  x11/xorg-libraries includes fontconfig in its
LIB_DEPENDS.  fontconfig sets USE_X_PREFIX in its Makefile, which causes
bsd.port.mk to set USE_XLIB, which then causes bsd.port.mk to add
X11.6:/usr/ports/x11/xorg-libraries to LIB_DEPENDS.

If I do a "make package-depends" in either of these two port
directories, I see a recursive make storm.