Improve usage of AC_INIT

Improve usage of AC_INIT

* configure.ac (AC_INIT): Add "GNU" in package, add bug address.
(PACKAGE_BUGREPORT): Use it.

* src/emacs.c (emacs_version): Use PACKAGE_VERSION rather than VERSION.
(emacs_bugreport): New variable.
(usage_message): Use PACKAGE_BUGREPORT.
(syms_of_emacs) <report-emacs-bug-address>: New variable.

* lisp/cus-start.el (report-emacs-bug-address): Set custom properties.

* lisp/mail/emacsbug.el (report-emacs-bug-address):
Variable is now defined in emacs.c.