configure: fix --with-foo=/path/to/foo

When a required tool is specified with --with-foo, store the variable
in the list.
(transplanted from 1d43b65599a4b869d83d4db67b9715b1298d74f7)

configure

@@ -121,6 +121,7 @@ check_for() {
                     eval val="\${${var}}"
                     if [ -n "${val}" ]; then
                         printf "${val} (cached)\n"
+                        add_to_var_list "${var}"
                         return 0
                     fi
                 fi