Warnings issued by the compiler are not errors. For example I also get warnings when I compile Bernstein's daemontools and djbdns on OpenBSD, but the compiler just continues and the programs run fine..
In your case the problem is
cc1: warnings being treated as errors. For something this specify to the NetBSD kernel, you probably get more response at one of the NetBSD mailing lists.
See
http://netbsd.org/mailinglists/