RFC: Reworking build flags

[Guillem Jover <guillem@debian.org>]

Hi!

As mentioned on the mail about the dpkg-build-api, buildflags and the
current interface they are exposed through have become problematic over
time. There are at least three areas which I think need to be reworked
or improved, which I'm listing below, and for which I'll send separate
mails to split the topics (two now and the last one later today or so):

 * «*_FOR_BUILD» flags handling
 * Multiple toolchains support
 * Versioned buildflags interface

If there are other problems or sub-items in these, I'm also interested,
but in any case I'd appreciate feedback on any of the above, as for
some I have a rough idea how to go about, which might be flawed, and
in other cases I'm not so sure.

Thanks,
Guillem