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

[Eli Zaretskii <eliz@gnu.org>]

> Cc: Spencer Baugh <sbaugh@janestreet.com>, 63556@debbugs.gnu.org
> Date: Thu, 18 May 2023 19:11:07 +0800
> From:  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:
> 
> > Thanks for reporting the problem. Since _Generic is used here merely
> > as a micro-optimization on 32-bit platforms, it's not worth whatever
> > porting hassle you've run into, so I installed the attached patch so
> > that modiff no longer uses _Generic.
> 
> Would anyone mind if I made configure check for that?

Depends on how complex is that, and what problem does it solve.

> 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)  \
>      ? code_for_signed_char			  \
>      : (sizeof (expression) == sizeof (short int) \
> 	? code_for_short_int			  \
> 	: (sizeof (expression) == sizeof (int)	  \
> 	   ? code_for_int			  \
> 	   ... and so on

AFAIR, sizeof is a compile-time expression, the preprocessor cannot
evaluate it.