Out of tree build broken for APR 1.7.x and 1.8.x

More precisely "make check" is broken for out of tree builds, although 
the core reason is not restricted to "make check". It was observed after 
r1909334 / r1909335, which introduce a dependency on 
arch/unix/apr_private.h to testpoll.c.

The header file arch/unix/apr_private.h (build tree) is generated from 
arch/unix/apr_private.h.in (source tree). But is contains

#include "../apr_private_common.h"

which will be looked up due to the relative path in the build tree, but 
actually is located in the source tree.

Currently arch/unix/apr_private.h is only used in testpoll.h, therefore 
the problem only shows up during make check. But in fact 
arch/unix/apr_private.h is broken for any potential use.

This problem is in 1.7.x and 1.8.x. I did not check apr trunk.

Unfortunately I have no idea how to make autoheader to use

#include "arch/apr_private_common.h"

instead of

#include "../apr_private_common.h"

in the generated arch/unix/apr_private.h.in.

Maybe someone has an idea?

Thanks and regards,

Rainer