* lib/strings.in.h: Include <locale.h>.
(strcasecmp_l): New declaration.
* lib/strcasecmp_l.c: New file, based on lib/strcasecmp.c.
* m4/strcasecmp_l.m4: New file.
* m4/strings_h.m4 (gl_STRINGS_H): Test for strcasecmp_l.
(gl_STRINGS_H_REQUIRE_DEFAULTS): Initialize GNULIB_STRCASECMP_L.
(gl_STRINGS_H_DEFAULTS): Initialize HAVE_STRCASECMP_L,
REPLACE_STRCASECMP_L.
* modules/strings-h (Makefile.am): Substitute GNULIB_STRCASECMP_L,
HAVE_STRCASECMP_L, REPLACE_STRCASECMP_L.
* modules/strcasecmp_l: New file.
* tests/test-strings-h-c++.cc: Check declaration of strcasecmp_l.
* doc/posix-functions/strcasecmp_l.texi: Mention the new module and the
macOS, Solaris, Cygwin bugs.