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>

 reporter_name="Alexander BIGGA"

 reporter_url="http://sourceware.org/ml/crossgcc/2008-07/msg00000.html"

 reporter_comment=""