Install and generate docs for gdb-add-index

Install and generate docs for gdb-add-index

The "gdb-add-index" script has been resurrected on:

  commit caf26be91a584ef141ac5d3cb31007731af8b8e3
  Author: Samuel Bronson <naesten@gmail.com>
  Date:   Fri Nov 15 16:09:33 2013 -0500

      Resurrect gdb-add-index as a contrib script

However, for some reason (I couldn't find it in the archives), only
the script has been checked-in; the Makefile parts responsible for
installing it in the system were left out.  This commit fixes that, by
also resurrecting the Makefile and documentation bits.

This commit is part of our effort to upstream the local Fedora GDB
changes.  With this commit, we'll only carry a very small
Fedora-specific modification to the script.

gdb/ChangeLog:
2017-01-12  Tom Tromey  <tom@tromey.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* Makefile.in (install-only): Install gdb-add-index.

gdb/doc/ChangeLog:
2017-01-12  Tom Tromey  <tom@tromey.com>
	    Sergio Durigan Junior  <sergiodj@redhat.com>

	* gdb.texinfo (Index Files): Mention gdb-add-index.
	(gdb-add-index man): New section.
	* Makefile.in (gdb-add-index.1): New rule to generate manpage
	from gdb.texinfo.