[rfc] autotools compatibility and Hurd as platform

[Justus Winter]

I made two tiny patches to enable notmuch to be built on the Hurd, the
first one is straight forward, it just adds GNU as a platform and
reuses the settings for Linux.

The second one deals with the problem that the value passed as an
argument to --build is not a triple but a tuple:

% dpkg-architecture -qDEB_HOST_GNU_TYPE
i486-gnu

notmuchs configure script expects the arguments for --host and --build
to be triple but autotools is much more forgiving here. It does so by
canonicalizing the values using some serious voodoo in
/usr/share/misc/config.sub.

The patch reuses config.sub to canonicalize the arguments, but this
introduces a build dependency on autotools and probably worse, it uses
autotools internals and the path to config.sub has to be hardcoded.

Since this might be not desirable we might also just drop the code
that parses the --host and --build options since we're not using them
anyway.

With these two patches notmuch builds fine and works on the Hurd, the
testsuite is running fine (modulo one test, but that might be broken,
haven't checked on linux) and the python bindings are fine too (I've
been successfully running afew to tag some mails).

Cheers,
Justus