configure: more autostuff compatibility

Ignore --program-prefix=...

Signed-off-by: "Yann E. MORIN" <yann.morin.1998@anciens.enib.fr>
(transplanted from d28b764334b8f8f3acfecafea3b52a6cc2e8a272)

reporter_name="Julien DUSSER"

reporter_url="http://sourceware.org/ml/crossgcc/2008-10/msg00054.html"

reporter_comment="Soft-float for the 405 PowerPC"