Changeset 909 for trunk/.release_year

Set the copyright year automatically in svndigest-copy-cache. The year
is stored in a file .release_year and AC_DEFINEd and AC_SUBSTed, so it
can be used in both svndigest-copy-cache and svndigest (via
config.h). No need files lib/copyright_year anymore. The file
.release_year is updated same fashion as copyright_year.cc was
previously. A difference is that a change in .release will cause
configure to be rerun, which is expensive but I think we can live with
the cost since it only happens once a year.