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.