Generate/update a version.h header file.

This is dedicated to Lubomir ;)

Should work (tm).  Not sure though if you want target
'gen_version_file' as a pre-requisite to $(NAME) or some other target.

[Dirk Hohndel: minor adjustments to make it work with gtk-gui.c]

Signed-off-by: Cristian Ionescu-Idbohrn <cristian.ionescu-idbohrn@axis.com>
Signed-off-by: Dirk Hohndel <dirk@hohndel.org>