Some code ported back to pre-C89 libraries where tolower (C) had
undefined behavior if C is not an upper case character.
Nowadays that function is _tolower which is itself obsolete,
and much Gnulib code already assumes this part of C89 anyway.
Assume C89 or better tolower, which simplifies the code
and should improve performance slightly.
* lib/mbmemcasecmp.c, lib/mbmemcasecoll.c, lib/mbscasecmp.c:
* lib/mbscasestr.c, lib/mbsncasecmp.c, lib/mbspcasecmp.c:
* lib/strcasecmp.c, lib/strcasestr.c, lib/strncasecmp.c:
(TOLOWER): Remove. All uses replaced by tolower.