fix handling of 3rd/4th args to AC_ARG_WITH()

--- a/configure.ac
+++ b/configure.ac
@@ -130,7 +130,7 @@ dnl See if we build X11
 AC_ARG_WITH(x11,
 AC_HELP_STRING([--with-x11],
     [Enable the Motif based X11 interface default=no]),
-    [with_x11=yes],[with_x11=no])
+    [with_x11=$withval])
   
 if test "x$with_x11" = "xyes"; then
   dnl Check for motif, error out if not found
@@ -144,7 +144,7 @@ dnl See if we build GTK
 AC_ARG_WITH(gtk,
 AC_HELP_STRING([--with-gtk],
     [Enable building the GTK Ski interface default=no]),
-    [with_gtk=yes],[with_gtk=no]
+    [with_gtk=$withval]
 )
 
 if test "x$with_gtk" = "xyes"; then