Fix compilation under GCC 4.9.1.

Binutils contains code that trips a warning in GCC 4.9.1 at the
configured warning levels here.  The upstream folks wouldn't have
noticed this, because Ubuntu is traditionally very conservative
about compiler versions, and is still shipping (iirc) 4.7.

More generally, setting werror inextricably ties one's build to
the toolchain used.

I've disabled werror for the binutils build.  Note that this script
actually builds binutils twice -- once for the standalone library,
and once inside GDB.