DGA_TRUE does not get defined, so the macro

        @DGA_TRUE@am__objects_1 = xf86dga.lo xf86dga2.lo

in xserver/hw/xfree86/dixmods/extmod/Makefile.in is replaced with

        #am__objects_1 = xf86dga.lo xf86dga2.lo

in xserver/hw/xfree86/dixmods/extmod/Makefile. So there is no defined function 
_XFree86DGAExtensionInit in xserver/hw/xfree86/dixmods/extmod/.libs/libextmod.dylib. When 
XDarwin is built, it is built also from xserver/mi/miinitext.c which has:

  330	#ifdef XF86VIDMODE
  331	extern void XFree86VidModeExtensionInit(INITARGS);
  332	#endif
  333	#ifdef XF86MISC
  334	extern void XFree86MiscExtensionInit(INITARGS);
  335	#endif
  336	#ifdef XFreeXDGA
  337	extern void XFree86DGAExtensionInit(INITARGS);
  338	#endif
... 
  620	#if !defined(PRINT_ONLY_SERVER) && !defined(NO_HW_ONLY_EXTS)
  621	#if defined(XF86VIDMODE)
  622	    if (!noXFree86VidModeExtension) XFree86VidModeExtensionInit();
  623	#endif
  624	#if defined(XF86MISC)
  625	    if (!noXFree86MiscExtension) XFree86MiscExtensionInit();
  626	#endif
  627	#if defined(XFreeXDGA)
  628	    if (!noXFree86DGAExtension) XFree86DGAExtensionInit();
  629	#endif
  630	#ifdef XF86DRI
  631	    if (!noXFree86DRIExtension) XFree86DRIExtensionInit();
  632	#endif
  633	#endif

So XDarwin can't be built because it has:

/sw/lib/odcctools/bin/ld: Undefined symbols:
_XFree86DGAExtensionInit
_XFree86MiscExtensionInit
_XFree86VidModeExtensionInit

And even if XDarwin would use xserver/hw/xfree86/dixmods/extmod/.libs/libextmod.dylib, 
_XFree86DGAExtensionInit would stay undefined ...

Mac OS X 10.4.8
GCC 4.0.1
xorg-server-1.1.99.3