NASM is more actively maintained and permits generating dependency information
as a sideeffect of assembling, thus cutting build times in half.

(Cherry-picked from libav commit 57b753b4)

Signed-off-by: James Almer <jamrial@gmail.com>

bc hasn't been needed since a982c5d7

Signed-off-by: James Almer <jamrial@gmail.com>
Signed-off-by: Luca Barbato <lu_zero@gentoo.org>

It is useful to support a future staging branch and to have an
automated consistency check on github pull requests.