Makefile: allow for different compilation methods

Makefile: allow for different compilation methods

* targets.mk: Remove targets compile-source and compile-single,
  obsoleted by new configuration option.  Remove repetitive code in
  clean targets by using pattern substitution.  Avoid superfluous
  invocations of find by using multiple path arguments.

* Makefile: Remove compile-source and compile-single target
  documentation.

* default.mk: Add new option _COMPILE_ to select compilation method.
  Set default value to keep current behaviour.

* lisp/Makefile: Use new $(_COMPILE_) to dispatch compilation target.
  Implement private targets compile-dirall (default), compile-single,
  compile-slint1 and compile-slint2.

The additional compilation methods catch more and/or different errors
in the sources by compiling the sources in a single Emacs process per
compilation and with different conditions of the source directory, but
take much longer to compile even in the absence of such errors.  The
default method to use can be changed (like all other options) via
local.mk or temporarily on the command line.