build: ensure release year in copyright notice is up-to-date

build: ensure release year in copyright notice is up-to-date

From a suggestion by Eric Blake.  See automake bug#11356.
This is a follow-up to previous patch 'v1.12-12-gb99b5be'.

* configure.ac (RELEASE_YEAR): New AC_SUBST'd variable, should hold
the value of the current year.
* Makefile.am (update-copyright): Be sure to also update the
definition of 'RELEASE_YEAR' in configure.ac.
* lib/Automake/Config.in ($RELEASE_YEAR): New exported variable,
initialized from the value substituted for '@RELEASE_YEAR@'.
(@EXPORT): Add it.
* automake.in, aclocal.in: Use '$RELEASE_YEAR' (which will be
substituted at make time) instead of hard-coding the release
year.  This should ensure the copyright range in the version
message and in the generated files (Makefile.in and aclocal.m4)
are automatically kept up-to-date.
* bootstrap (dosubst): Update, also substitute '@RELEASE_YEAR@'.

Signed-off-by: Stefano Lattarini <stefano.lattarini@gmail.com>