documentation: --with-system-libs also active for libexttextcat and libvisio

documentation for commit 25c27aae73f09759f7d6d874c54c6071e21d0c42
diff --git a/configure.in b/configure.in
index 9700de9..3b48331 100644
--- a/configure.in
+++ b/configure.in
@@ -685,7 +685,7 @@ AC_ARG_WITH(external-thes-dir,
AC_ARG_WITH(system-libs,
    AS_HELP_STRING([--with-system-libs],
        [Use libraries already on system -- enables all --with-system-* flags except
         mozilla, libvisio, libexttextcat and translate-toolkit.]),
         mozilla and translate-toolkit.]),
,)

AC_ARG_WITH(system-headers,