Including <locale.h> unconditionally in Emacs

[Paul Eggert]

<locale.h> was standardized in C89, and all C platforms in current use 
have it, if only to supply a dummy implementation with only a C locale. 
However, the Emacs C source code currently attempts to port to platforms 
lacking <locale.h> with code like this (though details vary):

 #ifdef HAVE_SETLOCALE
 # include <locale.h>
 #endif

This sort of code dates back to the 1990s when some older platforms 
predating C89 support presumably lacked <locale.h> and setlocale.

As Emacs surely long ago stopped porting to platforms so obsolete that 
they lack <locale.h>, I assume it would be OK for Emacs to start 
including <locale.h> unconditionally. I thought I'd ask here first, 
though, just to make sure.

This topic came up on the Gnulib mailing list, since Emacs uses Gnulib 
modules that currently have code to work around the absence of 
<locale.h>, code that is a bit of a hassle and that we'd rather stop 
maintaining and that is present only because Emacs uses those modules. 
You can see the Gnulib thread here:

https://lists.gnu.org/r/bug-gnulib/2024-06/msg00057.html