bug#11786: Use c_strcasecmp for portable ASCII case-insensitive comparison

[Eli Zaretskii]

> From: Andreas Schwab <schwab@linux-m68k.org>
> Cc: Paul Eggert <eggert@cs.ucla.edu>,  Dmitry Antipov <dmantipov@yandex.ru>,  
> 11786@debbugs.gnu.org
> Date: Tue, 26 Jun 2012 18:25:03 +0200
> 
> Eli Zaretskii <eliz@gnu.org> writes:
> 
> > Can tolower fail for these, somehow, in a locale that uses a multibyte
> > encoding for these strings, and if so, can you give a practical
> > example of such a failure?
> 
> tolower ('I') in a turkish locale.

OK, but that's not an issue with multibyte locales.

Anyway, I think having a custom replacement for tolower, perhaps using
the code of c_tolower from gnulib or its equivalent, will resolve
this.  Am I missing something?