Muffle GCC warnings in various ways.

Muffle GCC warnings in various ways.

Some don't require much in the way of contortion to muffle without using
GCC-specific tricks.  Others are hard or impossible to avoid -- because
they're to do with macro definitions, or even actually GCC bugs (e.g.,
the array-bounds warning in `square.c') -- without just muffling the
warnings explicitly, by name.