build: avoid stdin stall with GNU AS probing.

a758c5e added probing for various tools, such as AS. Unfortunately, GNU
AS is reading stdin with -v, and thus configure is stalled with
configure arguments such as --as=as.

Fixes Ticket #1898.
avformat/dv: about 3 times faster probe

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>