The problem seems to be in mk/tools/replace.mk.
The following patch does the trick for me:

--- mk/tools/replace.mk.orig    Sun Oct  2 09:36:07 2005
+++ mk/tools/replace.mk Sun Oct  2 09:42:00 2005
@@ -691,7 +691,10 @@
 TOOLS_DEPENDS.imake?=          xorg-imake>=6.8:../../x11/xorg-imake
 TOOLS_FIND_PREFIX+=            TOOLS_PREFIX.imake=xorg-imake
 TOOLS_PATH.imake=              ${TOOLS_PREFIX.imake}/${X11ROOT_PREFIX}/bin/imake
-.    else # !empty(X11_TYPE:Mnative)
+# !empty(X11_TYPE:Mnative)
+.    elif defined(IMAKE)
+TOOLS_PATH.imake=              ${IMAKE}
+.    else
 TOOLS_PATH.imake=              ${X11BASE}/bin/imake
 .    endif
 .  endif

What do you think?

-- 
Georg Schwarz    http://home.pages.de/~schwarz/
 georg.schwarz@freenet.de  +49 178 8545053