Problem building x11/gtk2 on older MacOS X

[John Ruschmeyer <jruschme%gmail.com@localhost>]

I recently set up an old Mac under Mac OS X 10.3.9 (Darwin 7.9). I initially encountered some issues with the age of the native X11 implementation and, so, have been building packages using the modular X packages.

I'm currently trying to build x11/gtk2, however, and seem to have it a blocker. Configuration proceeds until I get to the following:

checking for x86 platform... no
checking for XOpenDisplay... no
configure: error: *** libX11 not found. Check 'config.log' for more details.
*** Error code 1

Stop.
bmake: stopped in /usr/pkgsrc/x11/gtk2

Checking config.log reveals the basic cause:

configure:23460: checking for XOpenDisplay
configure:23460: gcc -o conftest -DGDK_PIXBUF_DISABLE_DEPRECATED -pipe -O2 -I/usr/pkg/include -I/usr/include -I/usr/pkg/include/freetype2 -D_REENTRANT -I/usr/pkg/include/python2.6 -Wall -DPREFIX="\"/usr/pkg\"" -DPNG_NO_ASSEMBLER_CODE -no-cpp-precomp -isystem /usr/include -I/usr/pkg/include -I/usr/include -I/usr/pkg/include/freetype2 -D_REENTRANT -I/usr/pkg/include/python2.6 -DG_DISABLE_SINGLE_INCLUDES -DATK_DISABLE_SINGLE_INCLUDES -DGDK_PIXBUF_DISABLE_SINGLE_INCLUDES -DGTK_DISABLE_SINGLE_INCLUDES -I/usr/pkg/include   -L/usr/pkg/lib -L/usr/lib -L/usr/pkg/lib/python2.6/config conftest.c  -L/usr/pkg/lib -lXext -lXrender -lX11 -lpthread -lintl -liconv >&5
ld: Undefined symbols:
_XauDisposeAuth
_XauGetBestAuthByAddr
_XdmcpWrap
configure:23460: $? = 1

Obviously, besides libX11, we also need libXau and libXdmcp. (This is confirmed by checking the buildlink files.) What I can't figure out is how to tell Configure that we need those libraries. 

Anybody seen this before? Any suggestions?

Thanks...
John