DIST OOT: use build_dir version.i file

DIST OOT: use build_dir version.i file

Requires the change of priority for include file in AM_CPPFLAGS and
that the version.h file include a "system header" version.i instead of a
local version.

Enable the passing of value from version.i to a OOT build done from a distribution
tarball. Enable a packager to touch files in custom_modifications and
generate a valid version.i file to be used in a OOT build from tarball.

Signed-off-by: Jonathan Rajotte <jonathan.rajotte-julien@efficios.com>
Signed-off-by: Michael Jeanson <mjeanson@efficios.com>
Signed-off-by: Jérémie Galarneau <jeremie.galarneau@efficios.com>