symlinkat: port to AIX 7.1 * doc/posix-functions/symlinkat.texi (symlinkat): Mention AIX porting problem. * lib/symlinkat.c: Always include errno.h. (rpl_symlinkat) [HAVE_SYMLINKAT]: New function. * lib/unistd.in.h (symlinkat): Add replacement machinery. * m4/symlinkat.m4 (gl_FUNC_SYMLINKAT): Check symlinkat behavior. * m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Add REPLACE_SYMLINKAT. * modules/symlinkat (Depends-on): Add fstatat if REPLACE_SYMLINKAT. (configure.ac): Also compile replacement if REPLACE_SYMLINKAT. * modules/unistd (unistd.h): Substitute REPLACE_SYMLINKAT.