* modules/pipe-posix: New file.
* m4/unistd_h.m4 (gl_UNISTD_H_DEFAULTS): Set default.
(gl_UNISTD_H): Check for declaration.
* modules/unistd (Makefile.am): Substitute it.
* lib/unistd.in.h (pipe): Provide it for mingw.
* doc/posix-functions/pipe.texi (pipe): Update documentation.
* MODULES.html.sh (File descriptor based Input/Output): Likewise.
Signed-off-by: Eric Blake <eblake@redhat.com>