Force pbuilder dependencies package installing (compiling libav for precise)

Hello everybody,
I'm trying to backport vivid (15.04) version of libav to precise (12.04).
The problem is that, when I'm trying to solve the dependencies with pbuilder, I'm having problems with libgnutls28-dev, as he can't replace the virtual package gnutls:
...
libgnutls28-dev : Conflicts: gnutls-dev which is a virtual package.
...

I can post a full build log if needed.
Here you can get dsc and other source files I used:
http://packages.ubuntu.com/source/vivid/libav

I've also already backported some needed packages, which you can find here:
https://launchpad.net/~itachi-san/+archive/ubuntu/ffmpeg

Hope someone can help me pointing out how to solve this pbuilder error...