We want to pass the version number to AC_INIT rather than
AM_INIT_AUTOMAKE. Since we can't pass shell variables to AC_INIT, just
define the version numbers in m4. We could just duplicate the version
into AC_INIT and keep the shell variables as is, but this is a little
cleaner, IMO. Later, we just recreate the DVDNAV_* shell variables
with the m4 values, and everything is the same as it was before.
Once we have the version being specified in AC_INIT, we can use the
new style AM_INIT_AUTOMAKE (in the next commit) where we specify
options for automake, such as the automake prerequisite, if we want to
create a bzipped tarball, etc.