Bug#738760: libav: Add proper raspberry pi CPU detection

[peter green]

Sebastian Ramacher wrote:
> there was a request to change the handling of the Raspberry Pi in the
> libav package. Could you please explain the changes applied to the
> Raspbian version? 
In raspbian we have a checker that runs after all our autobuilds (and I 
manually run a similar check when I do manual builds) that looks for 
files tagged as armv7 (note: it seems using neon compiler options causes 
files to be tagged as armv7 even if the CPU options aren't changed) and 
prevents the autobuilder from uploading them.

In general i've been adopting the principle that having armv7 code in 
raspbian is at best a pointless waste of space (if the code in question 
is correctly guarded behind runtime CPU detection) and at worst a 
serious problem (if it isn't). Therefore I have been responding to cases 
like this by simply disabling the armv7/neon code. The raspbian libav 
package got this treatment.

It's possible that I dodn't selct the best combination of flags, I'm no 
libav expert.

I have not tested whether an unodified debian libav package built in a 
raspbian chroot actually works on the Pi or not.