Here's a quick fix/patch:
Index: cad/xcircuit/patches/patch-configure_in
===================================================================
RCS file: cad/xcircuit/patches/patch-configure_in
diff -N cad/xcircuit/patches/patch-configure_in
--- /dev/null 1 Jan 1970 00:00:00 -0000
+++ cad/xcircuit/patches/patch-configure_in 22 Oct 2011 16:00:28 -0000
@@ -0,0 +1,11 @@
+$OpenBSD$
+--- configure.in.orig Sat Oct 22 11:43:35 2011
++++ configure.in Sat Oct 22 11:45:29 2011
+@@ -734,6 +734,7 @@ if test "x$xc_with_tcl" != "x" ; then
+ fi
+ fi
+
++SHDLIB_EXT=".so.${TK_LIB_FILE##*.so.}"
+ if test "x$xc_with_tcl" != "x" ; then
+ if test "x${xc_with_tk_libraries}" != "x" ; then
+ for libname in \
