C23 support in Autoconf

[Paul Eggert]

I installed the attached patch into Savannah master to get the ball 
rolling on Autoconf supporting C23.

Most C23 compatibility gizmos are now done in Gnulib. However, 
Autoconf's AC_PROG_CC needs to be changed to tack --std=gnu23 onto $CC. 
Although this may cause a few backward compatibility hassles with 
ancient packages that still use K&R C functions, C23 means these 
packages need to be changed to use function prototypes anyway.

Comments welcome.

0001-AC_PROG_CC-now-tries-C23-too.patch
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