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.

README

@@ -8,6 +8,16 @@ Bison requires GNU m4 1.4.6 or later.  See:
 
       https://ftp.gnu.org/gnu/m4/m4-1.4.6.tar.gz
 
+** Relocatability
+If you pass '--enable-relocatable' to 'configure', Bison is relocatable.
+
+A relocatable program can be moved or copied to a different location on the
+file system.  It can also be used through mount points for network sharing.
+It is possible to make symlinks to the installed and moved programs, and
+invoke them through the symlink.
+
+See "Enabling Relocatability" in the documentation.
+
 ** Internationalization
 Bison supports two catalogs: one for Bison itself (i.e., for the
 maintainer-side parser generation), and one for the generated parsers (i.e.,