configure: fix --with-prog=[...]

check_for didn't set variable 'where' when the path to a prog
was passed manually "(cached)".

Signed-off-by: "Titus von Boxberg" <titus@v9g.de>
(transplanted from b1be254591e7b524a0b06a2247a9331ebe0faf1d)

configure

@@ -171,6 +171,7 @@ check_for() {
                 eval val="\${${var}}"
                 if [ -n "${val}" ]; then
                     status="${val} (cached)\n"
+                    where="${val}"
                     break
                 fi
             fi