strings: Don't assume <strings.h> exists. * lib/strings.in.h: Include <strings.h> only if HAVE_STRINGS_H is 1. * m4/strings_h.m4 (gl_HEADER_STRINGS_H_BODY): Set HAVE_STRINGS_H. * modules/strings (Makefile.am): Substitute HAVE_STRINGS_H. * doc/posix-headers/strings.texi: Mention the MSVC problem.