Up to now, it has not been fully documented what a package from the base 
system needs to provide a "builtin" implementation of a pkgsrc package.

For example, on many platforms the binutils are (or were) considered 
builtin, although they are missing many of the header files and libraries.

The same goes for ncurses, where you need to explicitly say 
USE_NCURSES=yes when you want ncurses. So there are three ways:

.include "../../mk/bsd.curses.mk"
# some curses implementation, probably from the base system

.include "../../devel/ncurses/buildlink3.mk"
# some curses implementation, probably ncurses

USE_NCURSES= yes
.include "../../devel/ncurses/buildlink3.mk"
# yes, I definitely want ncurses and I mean it.

All this looks a little weird to me. To clarify the situation we should 
define the requirements for allowing a builtin (native) implementation 
to replace the pkgsrc package.

Roland