Makefile: Put VERSION info into version.texi rather than using -D

Unfortunately some older versions of makeinfo don't correctly
handle the -D command line option and fail to set the variable.
This then causes them to complain
 docs/qemu-ga-ref.texi:41: warning: undefined flag: VERSION

Work around this by doing as the autotools do, and writing
the information into a version.texi file which we then
include from the .texi files that need it.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Eric Blake <eblake@redhat.com>
Message-Id: <1487357968-31000-1-git-send-email-peter.maydell@linaro.org>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Markus Armbruster <armbru@redhat.com>