* gl/m4/strndup.m4 (gl_HEADER_STRING_H_DEFAULTS): Don't use gnulib's
strings.h replacement for now, it is too complicated to add right now.
* gl/lib/xstrndup.c: Instead of using gl_HEADER_STRING_H_DEFAULTS,
declare strndup here on systems which don't have it.
* gl/lib/strndup.c: Instead of using gl_HEADER_STRING_H_DEFAULTS,
declare strnlen here on systems which don't have it.
* gl/lib/strnlen.c: Import from gnulib.