version-etc: extend for packagers

version-etc: extend for packagers

Add three new configure options, intended for packagers:
  --with-packager="packager name"
  --with-packager-version="packager-specific version"
  --with-packager-bug-reports="packager bug reporting"

An example with coreutils:

  $ ./configure \
    --with-packager=Gentoo \
    --with-packager-bug-report=http://bugs.gentoo.org/ \
    --with-packager-version="patchset 1.6"

  $ ./src/ls --version | head -n3
  ls (GNU coreutils) 7.1-dirty
  Packaged by Gentoo (patchset 1.6)
  Copyright (C) 2009 Free Software Foundation, Inc.

Note that the bug reporting info via --help doesn't show up because
coreutils uses its own custom emit_bug_reporting_address() implementation
in src/system.h.  If it didn't, it'd look like:

  $ ./src/ls --help | tail -n4
  Report bugs to <bug-coreutils@gnu.org>.
  Report Gentoo bugs to <http://bugs.gentoo.org/>.
  GNU coreutils home page: <http://www.gnu.org/software/coreutils/>.
  General help using GNU software: <http://www.gnu.org/gethelp/>.

* lib/version-etc.c: Print new information, if provided.
* m4/version-etc.m4: New file.
* modules/version-etc (Files): Add m4/version-etc.m4.
(configure.ac): Add gl_VERSION_ETC.

Signed-off-by: Mike Frysinger <vapier@gentoo.org>