configure.ac: add -Werror to default compiler options

[Dan Kegel]

Juan wrote:
> Since Rob was noncommittal about whether -Werror was a good or a bad
> idea, I'll take a stance:  I think it's a bad one. ...
> some warnings are just plain wrong.  In some cases the "fix"
> to quiet the warning is uglier than the warning.

In those cases, we can disable the warning.

> Having a code base that has hundreds of warnings is also a problem,
> but we're fortunate enough not to be in that situation.

Are you sure about that?  When I configure with -Wall -Werror,
I can't even configure properly; one gets the errors

configure: libxrandr development files not found, XRandr won't be supported.
configure: libxinerama development files not found, multi-monitor
setups won't be supported.
configure: WARNING: Old Mesa headers detected. Consider upgrading your
Mesa libraries.
OpenGL and Direct3D won't be supported.

And if one blows past those, "make depend" fails.  In fact, everything
fails pretty spectacularly.

We should probably have a look at -Wall errors in more detail and breadth
before we assume that we're ok.
- Dan