Log Message:
* If PLIST_SRC is explicitly set to an empty value in a package Makefile,
  then automatically generate a PLIST that says "${PKGNAME} has no files".

* If PLIST_SRC and GENERATE_PLIST are not set in a package Makefile,
  and no PLIST files exist, then fail during the package build with
  PKG_FAIL_REASON.

* Remove "intentionally empty" PLISTs again.

Now, the easy way to say that a package installs no files is to just
add the following to the package Makefile:

	PLIST_SRC=	# empty