bug#63556: 29.0.90; Use of _Generic breaks Emacs build on GCC <4.9

[Po Lu via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

Paul Eggert <eggert@cs.ucla.edu> writes:

> There would be problems in doing that. GCC and Clang both have
> problems with _Generic (suggested by this original bug report), and
> some of them have to do with which warnings you've enabled. The
> configure-time check would run without the warnings whereas the build
> would run with them.
>
> This is partly why intprops-internal.h has given up on _Generic with
> GCC and Clang; see its ifdefs. I assumed that these problems didn't
> apply to their simple use in elogb, but apparently I assumed
> incorrectly.

[...]

>> Btw, couldn't the macro be defined to something along the lines of this
>> when the compiler only supports C99?
>>    #define foo(expression)			  \
>>      (sizeof (expression) == sizeof (signed char)  \
>
> That wouldn't work on unusual platforms that have padding bits. Of
> course we could simply refuse to port to targets like that; still, I'd
> be reluctant to do that just for this little issue.

How does intprops.h work on such systems?

> I couldn't measure the slowdown in this case. Until it's measurable I
> wouldn't worry about it. To some extent I put in that _Generic
> originally more as documentation than as an actual attempt to speed
> things up.

OK, but that's the second use of `long long' in src/ outside of
NT-specific files.  I can't help but be a little skittish.