[glibc] intl: Treat C.UTF-8 locale like C locale (BZ# 16621)

[Florian Weimer]

* Andreas Schwab via Libc-alpha:

> On Sep 04 2023, Florian Weimer via Glibc-cvs wrote:
>
>> diff --git a/intl/dcigettext.c b/intl/dcigettext.c
>> index 7886ac9545..27063886d2 100644
>> --- a/intl/dcigettext.c
>> +++ b/intl/dcigettext.c
>> @@ -1560,8 +1560,12 @@ guess_category_value (int category, const char *categoryname)
>>       2. The precise output of some programs in the "C" locale is specified
>>  	by POSIX and should not depend on environment variables like
>>  	"LANGUAGE" or system-dependent information.  We allow such programs
>> -        to use gettext().  */
>> -  if (strcmp (locale, "C") == 0)
>> +        to use gettext().
>> +     Ignore LANGUAGE and its system-dependent analogon also if the locale is
>> +     set to "C.UTF-8" or, more generally, to "C.<encoding>", because that's
>> +     the by-design behaviour for glibc, see
>> +     <https://sourceware.org/glibc/wiki/Proposals/C.UTF-8>.  */
>> +  if (locale[0] == 'C' && (locale[1] == '\0' || locale[1] == '.'))
>>      return locale;
>
> Should this also cover the "POSIX" locale?

"POSIX.UTF-8" is not a valid locale string, as far as I can tell, so we
do not have to treat "POSIX" differently in this context.  Plain "POSIX"
seems to have been normalized to "C" at this point.

> What about the reference to "C" in dcigettext?

The remaining references allow having different messages for C and
C.UTF-8, maybe with added Unicode punctuation characters.  That seems
useful.

Thanks,
Florian