maint: generate ChangeLog from git log

maint: generate ChangeLog from git log

Following the practice set by various other GNU projects, we start
to automatically generate the ChangeLog file from the git commit
messages.  This will avoid duplication (as the ChangeLog entries
were always inserted both in the git commit message and in the
version-controlled ChangeLog file), and potential problems with
spurious merge conflicts (which, although greatly mitigated by
Bruno Haible's `git-merge-changelog' helper program, have never
been completely solved).

* ChangeLog: Moved ...
* ChangeLog.3: ... to this.
* build-aux/gitlog-to-changelog: New script, synced from gnulib.
* cfg.mk (gnulib-update): Also sync gitlog-to-changelog.
* Makefile.am (gen-ChangeLog): New .PHONY rule, generate the
ChangeLog for distribution.
(dist-hook): Depend on it.
($(srcdir)/ChangeLog): New dummy rule, to pacify automake "gnu"
strictness.  Creates a dummy ChangeLog, that will be overridden
by the proper one at distribution time.
(gen_start_date): New variable, the date starting from which the
git log entries are to be copied in the generated ChangeLog.
(EXTRA_DIST): Add ChangeLog.3 and gitlog-to-changelog.
* configure.ac (AC_CONFIG_SRCDIR): Use 'lib/autoconf/autoconf.m4'
instead of 'ChangeLog' as the sentinel file.
* .gitignore: Add ChangeLog.