Recent advances in cross building src:perl

[Niko Tyni <ntyni@debian.org>]

Hi,

I've been working on cross build support for src:perl, which has
been a sore point for quite some time. Current results are on the
ntyni/cross-5.22 branch on alioth (but might still get rebased/rewritten):

 https://anonscm.debian.org/cgit/perl/perl.git/log/?h=ntyni/cross-5.22

Summary: Configure still needs runtime probes, but the resulting probed
configuration data can be fed into a cross build with a separate binary
package, currently called perl-config-data. This is the same approach that
Neil Williams worked on with src:perl-cross-debian a couple of years ago,
but we now build the package from src:perl itself.

Apart from that, I've got cross builds fully working (tested on amd64 ->
mipsel) with crossbuild-essential et al. from sid. Many thanks to those
who have contributed to getting this stuff working in Debian, it has
all become a lot easier than it used to be.

Bootstrapping a new architecture could be achieved by manually preparing
the configuration data package, probably by modifying one for an existing
similar architecture. I'm open to suggestions on how to make this as
easy as possible.

Thanks to past upstream work by Jess Robinson and Neil, this level of
cross build support needed only three upstream changes, one of which
is trivial and another (podlators) already merged.  People wanting to
see the remaining issue of Configure cross-compiling support fixed are
invited to read https://rt.perl.org/Ticket/Display.html?id=124326 and
volunteer to help upstream.

As a proof of concept, I've also implemented a 'stage1' build profile
that only builds the perl-config-data binary package. However, I'm not
really sure how useful that is. The idea there is that a functional but
slow Debian architecture could run just the configuration probes natively
(or reuse old configuration data on minor upgrades), and the rest could
be cross built. I suspect this doesn't help real bootstrapping, as the
various dpkg-dev tools require a working /usr/bin/perl to put any package
together in the first place.

@Dominic and others on the src:perl side: the commits before the cross
related upstream changes are intended to be non-disruptive and could
(at least in theory) be merged into unstable straight away. Eyeballs
would be very welcome; the debian-cross list should probably be dropped
from that discussion.

I'm not sure if it's viable to upload this into experimental at
this point; the buildds probably don't have #809730 fixed yet,
do they?  Without that fix, sbuild applies the cross build dependency
on perl-config-data to native builds too and bails out straight away.
Also, we'll be needing experimental for staging 5.24 in a few months so
maybe it would be better to use a custom repository for testing this?

In the long run, I can see cross building eventually replacing our
current somewhat half-baked new architecture bootstrapping support that
is the reason we need to roll our own horrific debian/rules and can't
use debhelper et al. I think I'd want to see how this stuff rebases on
a couple of upstream releases first before declaring it stable, though.

In any case, I'd be interested in comments and thoughts on this.
Is it useful, or just a worthless workaround as long as the Configure
issues remain unsolved?
-- 
Niko Tyni   ntyni@debian.org