[v3] add -fno-exceptions support

[Jason Merrill]

>>>>> "Benjamin" == Benjamin Kosnik <bkoz@redhat.com> writes:

> This patch allows one to compile libstdc++-v3 with -fno-exceptions, if
> so desired.

Thanks!

> I'm posting this for comments only: __EXCEPTIONS is still broken so I
> can't really be sure that nothing breaks with this.

I'll check in a fix today.

> 	* libsupc++/new_op.cc (new): Include functexcept.h. Use
> 	std::__throw_bad_alloc() instead of throw bad_alloc.

This is wrong.  Nothing in libsupc++ should depend on anything but
libsupc++, libgcc and (as little as possible) the C library.  Similarly,
std::__throw_bad_cast should call __cxa_bad_cast, not the other way around.

Jason