Makefile: enable one-process-per-source compilations again

Makefile: enable one-process-per-source compilations again

* default.mk: Re-introduce ELC for single file compilation.

* targets.mk: New targets compile-single and compile-single-source
  that get handed off to lisp/Makefile.

* lisp/Makefile: Implement pattern rule using $(ELC).  Implement
  target compile-single that uses a separate Emacs process for each
  target by invoking the pattern rule.  Implement target
  compile-single-source that additionally removes the compiled file
  directly after it has been produced, so compilation will always use
  source files only, never byte-compiled files.

* Makefile: Document new targets.

These new targets are meant to check for problems with
interdependencies in Org.