Update program --help output to match current GCS.

Update program --help output to match current GCS.

* configure.ac: Set and substitute PACKAGE_URL if AC_PACKAGE_URL
is not defined, for compatibility to Autoconf < 2.64.
* Makefile.am (do_subst): Substitute PACKAGE_BUGREPORT and
PACKAGE_URL.
(sc_diff_automake_in_automake): Update number of diff lines for
additional substitutions.
* aclocal.in (usage): Use PACKAGE_BUGREPORT.  Point to Automake
home page and GNU general help page.
* automake.in (usage): Likewise.
* doc/automake.texi: New flag PACKAGE_BUGREPORT, to factor email
address.
(Introduction, Creating amhello, amhello Explained, Options):
Use it throughout.
* lib/Automake/Makefile.am (do_subst): Substitute
PACKAGE_BUGREPORT.
* lib/Automake/Config.in ($PACKAGE_BUGREPORT): New global.
* lib/Automake/ChannelDefs.pm: Use it for footer of fatal
messages.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>