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.

src/yacc.in

@@ -0,0 +1,22 @@
+#! /bin/sh
+
+@relocatable_sh@
+if test "@RELOCATABLE@" = yes; then
+  exec_prefix="@exec_prefix@"
+  bindir="@bindir@"
+  orig_installdir="$bindir" # see Makefile.am's *_SCRIPTS variables
+  func_find_curr_installdir # determine curr_installdir
+  func_find_prefixes
+  relocate () {
+    echo "$1/" \
+    | sed -e "s%^${orig_installprefix}/%${curr_installprefix}/%" \
+    | sed -e 's,/$,,'
+  }
+else
+  relocate () {
+    echo "$1"
+  }
+fi
+
+bindir=`relocate "@bindir@"`
+exec "$bindir/bison" -y "$@"