Include "config.h" instead of "private/config.h" on HAVE_CONFIG_H

Include "config.h" instead of "private/config.h" on HAVE_CONFIG_H
(Change the behavior of HAVE_CONFIG_H macro to the standard one
which assumes that no folder is specified in #include "config.h".)

* configure.ac (AM_CONFIG_HEADER): Replace include/private/config.h to
include/config.h.
* cord/cordbscs.c: Include "config.h" (instead of "private/config.h")
if HAVE_CONFIG_H defined.
* cord/cordprnt.c: Likewise.
* cord/cordxtra.c: Likewise.
* gc_cpp.cc: Likewise.
* include/private/gc_pmark.h: Likewise.
* include/private/gc_priv.h: Likewise.
* real_malloc.c: Likewise.
* tests/disclaim_test.c: Likewise.
* tests/initsecondarythread.c: Likewise.
* tests/subthread_create.c: Likewise.
* tests/test.c: Likewise.
* tests/test_cpp.cc: Likewise.
* tests/thread_leak_test.c: Likewise.
* tests/threadkey_test.c: Likewise.