What is the proper configuration of sbuild for backports ?

[Charles Plessy <plessy@debian.org>]

Dear all,

I am preparing a backport for python-pysam, which will build-depend
on samtools version 0.1.19.  A backport of samtools (0.1.19-1~bpo70+1)
exists in the archive (thanks for the fast processing !).

Now I am a bit confused on how to build this backport-depending backport,
and could not find information on line.  Perhaps I used the wrong keywords ?
Anyway, I devised two possible solutions:

1) pin wheezy-backports to a priority of 500.  The problem is that it causes
   the chroot itself to upgrade linux-libc-dev and tar, which I am not sure is
   wanted.

2) Use aptitude as a resolver.  This solves the problem on my local machine, but
   I do not know if it will work on Debian's buildds (for which I did not manage
   to find the configuration).

Any advice ?

Have a nice week-end,

-- 
Charles Plessy
Debian Med packaging team,
http://www.debian.org/devel/debian-med
Tsurumi, Kanagawa, Japan