[FFmpeg-soc] Allow out of source tree builds for libavfilter

[Vitor]

Hi

Víctor Paesa wrote:
> Hi,
> 
> I'd like to propose the attached patch to $subject

Since Diego who knows the build system pretty well ok'ed it, I think you 
can safely commit it. If you don't want to bother asking a svn account 
in SoC, I can just commit it for you.

Diego: do you see anything else obviously wrong at 00_build.diff?

Thank you two for having a look at it.
-Vitor