Log Message:
Rework emacs.mk:

1) Simplify the way how an emacs version is picked when no emacs
   is installed, but a user try to install an Emacs Lisp package.
   Just pick up the version set as EMACS_TYPE than searching for
   versions already installed etc.  If the EMACS_TYPE version is
   not supported by the Emacs Lisp Package, just fail.  EMACS_TYPE
   be default to GNU Emacs 21.

   (In other words, users should set EMACS_TYPE as they want.
   Otherwise GNU Emacs 21 is used.)

2) All Emacs Lisp Packages *must* prepend EMACS_PKGNAME_PREFIX to
   a) the PKGNAME itself, and b) PKGNAME in its dependency lines.
   EMACS_PKGNAME_PREFIX is expanded to "xemacs-" when XEmacs is
   used.  This keeps dependency graph of Emacs-Lisp-packages-
   installed-for-XEmacs consistent.

3) Document EMACS_* variables as much as possible.

4) Provide more cookies for PLIST.  Maybe utilized later.

Note that the 2) change doesn't affect the default, GNU Emacs 21
behaviour.  So no version / revision bumps in this commit.