* lib/strings.in.h (strncasecmp_l): New declaration.
* lib/strncasecmp_l.c: New file, based on lib/strncasecmp.c.
* m4/strncasecmp_l.m4: New file.
* m4/strings_h.m4 (gl_STRINGS_H): Test for strncasecmp_l.
(gl_STRINGS_H_REQUIRE_DEFAULTS): Initialize GNULIB_STRNCASECMP_L.
(gl_STRINGS_H_DEFAULTS): Initialize HAVE_STRNCASECMP_L,
REPLACE_STRNCASECMP_L.
* modules/strings-h (Makefile.am): Substitute GNULIB_STRNCASECMP_L,
HAVE_STRNCASECMP_L, REPLACE_STRNCASECMP_L.
* modules/strncasecmp_l: New file.
* tests/test-strings-h-c++.cc: Check declaration of strncasecmp_l.
* doc/posix-functions/strncasecmp_l.texi: Mention the new module and the
macOS, Solaris, Cygwin bugs.