Log Message:
Split out the code that deals with checking whether the software is
built-in or not into a separate builtin.mk file.  The code to deal
checking for built-in software is much simpler to deal with in pkgsrc.

The buildlink3.mk file for a package will be of the usual format
regardless of the package, which makes it simpler for packagers to
update a package.

The builtin.mk file for a package must define a single yes/no variable
USE_BUILTIN.<pkg> that is used by bsd.buildlink3.mk to decide whether
to use the built-in software or to use the pkgsrc software.