New module 'strerror_r-posix'.

New module 'strerror_r-posix'.

* lib/string.in.h (strerror_r): New declaration.
* lib/strerror_r.c: New file.
* m4/strerror_r.m4: New file.
* m4/string_h.m4 (gl_HEADER_STRING_H_BODY): Check for the declaration
of strerror_r.
(gl_HEADER_STRING_H_DEFAULTS): Initialize GNULIB_STRERROR_R,
HAVE_DECL_STRERROR_R, REPLACE_STRERROR_R.
* modules/strerror_r-posix: New file.
* modules/string (Makefile.am): Substitute GNULIB_STRERROR_R,
HAVE_DECL_STRERROR_R, REPLACE_STRERROR_R.
* doc/posix-functions/strerror_r.texi: Mention the new module and the
portability problems.