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="Thomas Petazzoni"

 reporter_url="http://www.free-electrons.com/"

 reporter_comment="Very basic testing only"