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/local.mk

@@ -13,10 +13,17 @@
 ## You should have received a copy of the GNU General Public License
 ## along with this program.  If not, see <http://www.gnu.org/licenses/>.
 
+CLEANDIRS += %D%/*.dSYM
+
 bin_PROGRAMS = src/bison
 # Prettify Automake-computed names of compiled objects.
 src_bison_SHORTNAME = bison
 
+src_bison_CPPFLAGS = $(AM_CPPFLAGS) -DINSTALLDIR=\"$(bindir)\"
+if RELOCATABLE_VIA_LD
+  src_bison_LDFLAGS = `$(RELOCATABLE_LDFLAGS) $(bindir)`
+endif
+
 src_bison_CFLAGS = $(AM_CFLAGS) $(WERROR_CFLAGS)
 src_bison_SOURCES =                             \
   src/AnnotationList.c                          \
@@ -127,16 +134,7 @@ src_bison_LDADD =                               \
 ## ------ ##
 
 if ENABLE_YACC
-  bin_SCRIPTS = src/yacc
+  nodist_bin_SCRIPTS = src/yacc
 endif
 EXTRA_SCRIPTS = src/yacc
 MOSTLYCLEANFILES += src/yacc
-CLEANDIRS += %D%/*.dSYM
-
-src/yacc:
-	$(AM_V_GEN)rm -f $@ $@.tmp
-	$(AM_V_at)$(MKDIR_P) src
-	$(AM_V_at)echo '#! /bin/sh' >$@.tmp
-	$(AM_V_at)echo "exec '$(bindir)/bison' -y "'"$$@"' >>$@.tmp
-	$(AM_V_at)chmod a+x $@.tmp
-	$(AM_V_at)mv $@.tmp $@