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.

README.md

@@ -12,14 +12,27 @@ If you want to hack on jq, feel free, but be warned that its internals
 are not well-documented at the moment. Bring a hard hat and a
 shovel. Also, read the wiki: http://github.com/stedolan/jq/wiki
 
+Source tarball and built executable releases can be found on the
+homepage and on the github release page, https://github.com/stedolan/jq/releases
+
 If you're building directly from the latest git, you'll need flex,
-bison, libtool, make, autoconf and libonig installed. To build, run:
+bison (3.0 or newer), libtool, make, and autoconf installed.  To get
+regexp support you'll also need to install Oniguruma (note that jq's
+tests require regexp support to pass).  To build, run:
 
     autoreconf -i   # if building from git
     ./configure
     make -j8
     make check
 
+To build without bison or flex, add `--disable-maintainer-mode` to the
+./configure invocation:
+
+    ./configure --disable-maintainer-mode
+
+(Developers must not use `--disable-maintainer-mode`, not when making
+changes to the jq parser and/or lexer.)
+
 To build a statically linked version of jq, run:
 
     make LDFLAGS=-all-static