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.

.gitignore

@@ -16,12 +16,6 @@ jq.1
 # Something delightfully recursive happens otherwise
 docs/content/2.download/source/*
 
-# Autogenerated
-lexer.c
-lexer.h
-parser.c
-parser.h
-
 # Autotools junk
 .libs
 .deps