Don't turn warnings on in Subsurface's build

We're getting a ton of them and they're mostly harmless. I've already
turned on the ones that are problematic (with -Werror even).

Signed-off-by: Thiago Macieira <thiago@macieira.org>
Turn off exceptions in C++

We don't use them in our code and Qt doesn't throw either, so save a
few bytes and maybe a few setjump() calls on Windows.

Signed-off-by: Thiago Macieira <thiago@macieira.org>