Add --disable-maintainer-mode; make bison optional

Also flex is now optional.

The outputs of flex and bison are now committed.  By default they get
built, but users who want to build from git can now

    ./configure --disable-maintainer-mode

to turn off the dependency on bison and flex.

Maintainers must, of course, commit the bison and/or flex outputs when
they make changes to parser.y and/or lexer.l, respectively.

setup.sh

@@ -13,7 +13,7 @@ if [ "clean" == "$1" ]; then
   find config -name m4 -prune -o -type f -print | xargs rm -f
   rm -rf tmp autom4te.cache
   rm -f INSTALL Makefile.in aclocal.m4 configure config.h.in ChangeLog
-  rm -f jv_utf8_tables.gen.h lexer.c lexer.h parser.c parser.h
+  rm -f jv_utf8_tables.gen.h
 elif [ "superclean" == "$1" ]; then
   # if autoconf errors during distcheck, it leaves files that need chmod'ing
   ver=$(scripts/version|tr -d '\n')