buildlink3.mk vs builtin.mk

["Larson, Timothy E." <TELarson%west.com@localhost>]

Some of my package's dependencies have a builtin.mk as well as a buildlink3.mk. 
 Should I include the former instead of the latter in my package's Makefile? 
Other packages' Makefiles have CHECK_BUILTIN.pkgname set around the include, 
but I can't find documentation describing this. I could try blindly following 
the examples, but I'd prefer to understand. Reading the Pkgsrc Guide is not 
very illuminating on this point.


Thanks,
Tim