configure.ac: add -Werror to default compiler options

[Dan Kegel]

OK, I just ran with -Wall -Werror, and got a grand total of four errors:

freetype.c:1051: warning: 'name.string_len' is used uninitialized in
this function
Looks bogus.

engine.c:2128: warning: 'str' may be used uninitialized in this function
Bogus.

context.c:80: warning: 'update_minfilter' may be used uninitialized in
this function
context.c:80: warning: 'update_magfilter' may be used uninitialized in
this function

Real!   Looks like one slipped by yesterday:
http://source.winehq.org/git/wine.git/?a=commitdiff;h=9533a5cbbf76cf4b9013ed90dd96e0f3995d44f3

That's not a bad haul.

Now, if everyone were building with -O2 -Wall -Werror, that would have been
caught sooner.   But hey, if Alexandre doesn't like the idea, I guess
it won't fly.

So my alternate suggestion is for patchwatcher to reject patches that
fail with -Werror.
(And work around the couple of bogus errors I listed above.)

That gets us the desired benefits without running into the problems
Alexandre and Marcus pointed out.

How's that sound?
- Dan