build: Move config.h to conf/config.h

* conf/: New dir.
* configure.ac (AC_CONFIG_HEADER): Move header to conf dir.
* doc/Makefile.am (mkdefsinc): Adjust rule.
* lang/python/Makefile.am (copystamp): Ditto.

C++ uses those stupid files without a suffix and thus the new standard
header <version> as included by newer other libc++ versions may
conflict with the VERSION file in out top directory. This change
should solve the problem.

• GnuPG-bug-id: T4168
• Signed-off-by: Werner Koch <wk@gnupg.org>