unistd: warn on use of environ without module

unistd: warn on use of environ without module

_GL_WARN_ON_USE only works for functions; so we wrap access to
environ through a function when CFLAGS has -DGNULIB_POSIXCHECK=1.

* modules/unistd (Depends-on): Add warn-on-use.
(Makefile.am): Provide new substitutions.
* m4/unistd_h.m4 (gl_UNISTD_H): Check for inline and environ.
* lib/unistd.in.h (environ): Wrap with a warning helper function.

Signed-off-by: Eric Blake <ebb9@byu.net>