Add support for autoconf macros in m4/

Needed by commit 972a34ec.

Use autoreconf in autogen.sh instead of the handwritten version,
it's simpler, and also updates things that we weren't handling.

Drop the hand-written INSTALL file.  Its information content was
~zero, and autotools wants to overwrite it with its own version,
so don't fight that, just .gitignore it.