configure: Move up the avbuild directory creation

The early check for inconsistent in-source vs out-of-source build
cannot generate a config.log otherwise.

(Cherry-picked from libav commit 0ee78020cd41d81eec651acd7fc65906207796f3)
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>
Signed-off-by: James Almer <jamrial@gmail.com>
