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)

reporter_name="Alexander BIGGA"

reporter_url="http://sourceware.org/ml/crossgcc/2008-06/msg00031.html"

reporter_comment=""