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

[Eli Zaretskii <eliz@gnu.org>]

> Date: Tue, 26 Jun 2012 21:04:24 +0400
> From: Dmitry Antipov <dmantipov@yandex.ru>
> CC: Paul Eggert <eggert@cs.ucla.edu>, 11786@debbugs.gnu.org
> 
> It was an attempt to get rid from the zoo of strcase(n)cmp implementations
> (one in etags.c, one in lwlib.c and next one in xfaces.c).

Thanks for the explanation.

I was talking only about xtrcasecmp, defined on xfaces.c.  lwlib.c can
use it, if we want.  As for etags, I don't care much, but pulling in
another bunch of gnulib modules just for its sake sounds gross to me.

> I assumed that these library functions, if available, should work
> correctly regardless from the locale on any non-ancient system.

It's actually the other way around: to DTRT, these library functions
_must_ be locale-dependent, because the collation sequence might be
different in each locale, even for locales that use the same Latin
characters.

> If that is not true, gnulib module may be a workaround.

We already had a workaround for that, so I see no reason to use that
of gnulib.