package: make bison a relocatable package

Suggested by David Barto
https://lists.gnu.org/archive/html/help-bison/2015-02/msg00004.html
and Victor Zverovich.
https://lists.gnu.org/archive/html/bison-patches/2018-10/msg00121.html

This is very easy to do, thanks to work by Bruno Haible in gnulib.
See "Supporting Relocation" in gnulib's documentation.

* bootstrap.conf: We need relocatable-prog and relocatable-script (for yacc).

* src/yacc.in: New.
* configure.ac, src/local.mk: Instantiate it.
* src/main.c, src/output.c (main, pkgdatadir): Use relocatable2.

* doc/bison.texi (FAQ): Document it.

THANKS

@@ -40,6 +40,7 @@ Dagobert Michelsen        dam@baltic-online.de
 Daniel Frużyński          daniel@poradnik-webmastera.com
 Daniel Galloway           dg1751@att.com
 Daniel Hagerty            hag@gnu.org
+David Barto               david.barto@sparqlcity.com
 David J. MacKenzie        djm@gnu.org
 David Kastrup             dak@gnu.org
 David Michael             fedora.dm0@gmail.com
@@ -174,6 +175,7 @@ Troy A. Johnson           troyj@ecn.purdue.edu
 Tys Lefering              gccbison@gmail.com
 Valentin Tolmer           nitnelave1@gmail.com
 Victor Khomenko           victor.khomenko@newcastle.ac.uk
+Victor Zverovich          victor.zverovich@gmail.com
 Vin Shelton               acs@alumni.princeton.edu
 W.C.A. Wijngaards         wouter@NLnetLabs.nl
 Wayne Green               wayne@infosavvy.com