Overhaul Makefile system

Overhaul Makefile system

* Makefile (DEFS): Use more-sophisticated Makefile.defs.default setup.
  (info-only): New rule to make just the info file.
  (install): Use improved install_info method.
  (dist, debprepare): Update rules for git.
  (elpa): New rule that makes an ELPA package for Planner.

* Makefile.defs.default: Rename from Makefile.defs.
  (install_info): New improved rule based on the old INSTALLINFO
  variable.
  (ELPADIR, ELPADESC): New variables used for making the Planner ELPA
  package.
  (DISTRIBUTOR): Must have forgotten to add this from the last time I
  modified Makefile.

* planner-el.texi (Advanced Installation): Update compilation
  instructions.