configure.ac: add -Werror to default compiler options

[Juan Lang]

> You're pretty negative on this idea, aren't you?  Let me whittle away at
> it a bit before we reject it as impractical.

Sure, have fun.

I've used -Wall -Werror with success in the past with a medium-sized
code base, and used it as a stick to keep people from checking in
dodgy code.  So it's not that it's always impractical.  The issue I
have is that a distributed development environment makes it hard, as
does maintaining a portable code base linked against many possible
versions of libraries.  If a commit doesn't break on Alexandre's box,
and it gets committed, but it does break on some other machine, what
then?  Who fixes it?

I think that individually, we can turn it on from time to time to see
what pops up, just like running with valgrind.  But forcing everyone
to run with it seems draconian.
--Juan