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.

Makefile.am

@@ -20,10 +20,18 @@ ACLOCAL_AMFLAGS = -I config/m4
 
 # While there is some autoconf macro support for lex/flex, it doesn't support
 # header file creation so we'll use good old make
+if MAINTAINER_MODE
 BUILT_SOURCES = lexer.h lexer.c parser.h parser.c version.h
 lexer.c: lexer.l
 	$(AM_V_LEX) flex -o lexer.c --header-file=lexer.h $<
 lexer.h: lexer.c
+else
+BUILT_SOURCES = 
+.y.c:
+	$(AM_V_YACC) echo "NOT building parser.c!"
+.l.c:
+	$(AM_V_LEX) echo "NOT building lexer.c!"
+endif
 
 # Tell YACC (bison) autoconf macros that you want a header file created.
 # If the --warnings=all fails, you probably have an old version of bison