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.

bootstrap.conf

@@ -35,6 +35,7 @@ gnulib_modules='
   quote quotearg
   readme-release
   realloc-posix
+  relocatable-prog relocatable-script
   spawn-pipe stdbool stpcpy strdup-posix strerror strverscmp
   timevar
   unistd unistd-safer unlink unlocked-io