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.

lib/.gitignore

@@ -294,3 +294,18 @@
 /bitset/
 /bitsetv.c
 /bitsetv.h
+/allocator.c
+/allocator.h
+/areadlink.c
+/areadlink.h
+/canonicalize-lgpl.c
+/careadlinkat.c
+/careadlinkat.h
+/progreloc.c
+/readlink.c
+/relocatable.c
+/relocatable.h
+/relocwrapper.c
+/setenv.c
+/xreadlink.c
+/xreadlink.h