Stop automatically adding warning flags and -Werror.

Stop automatically adding warning flags and -Werror.

It's better to be lax for normal users trying to build the puzzles
from source to actually run them. That way, warning changes in some
particular compiler I haven't seen yet won't break the build.

Instead, I've invented a cmake setting -DSTRICT=ON which turns on all
those flags. So I can build with them myself, to ensure the code is as
portable as possible. And that flag is set in Buildscr, so that my
official builds won't complete until that warning mode is satisfied.