Bug#672927: apt-get build-dep -a uses build arch for [] restrictions, not host arch

[Wookey <wookey@wookware.org>]

Package: apt
Version: 0.9.3
Severity: Normal

ncurses build-depends on:
 g++-multilib [amd64 i386 kfreebsd-amd64 powerpc ppc64 s390 sparc]

But apt-get build-dep -a armel tries to install the host-arch
version, even though it is not needed on armel. 

So, apt is using the wrong arch to check the [] qualifiers.

This bug is a problem because it prevents cross-builds of various
low-level packages such as zlib and ncurses (and thus also things build on
top of them when bootstrapping, which is nearly everything).

# apt-get build-dep -a armel ncurses
E: Build-Depends dependency for ncurses cannot be satisfied because
the package g++-multilib cannot be found

# apt-cache policy g++-multilib:armel
N: Unable to locate package g++-multilib:armel
N: Couldn't find any package by regex 'g++-multilib'

# apt-cache policy g++-multilib
g++-multilib:
  Installed: (none)
  Candidate: 4:4.7.0-6
  Version table:
    4:4.7.0-6 0
       500 http://ftp.uk.debian.org/debian/ unstable/main amd64 Packages

There is a corresponding Ubuntu bug:
https://bugs.launchpad.net/ubuntu/+source/apt/+bug/970050

Wookey