build: pull in conditional headers during GNULIB_POSIXCHECK

build: pull in conditional headers during GNULIB_POSIXCHECK

Any time we conditionally include a header in order to satisfy
prerequisites of one module, we must also include that header
under GNULIB_POSIXCHECK to avoid compilation errors if that
module is not in use.  If this is not done, then our link warning
override can occur before the system's declaration, which
causes a syntax error when the system header is finally included.

* lib/stdio.in.h (includes): Using GNULIB_POSIXCHECK also requires
definitions from any conditionally-included headers.
* lib/stdlib.in.h (includes): Likewise.
* lib/unistd.in.h (includes): Likewise.

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