Any ideas or examples of building one package that depends on source code
from another package?

Should I just unextract the other distfile to the local WRKDIR or is it
okay to extract that other source in its own?

This is what I have working now (for my security/iptables):

do-build:
        cd ${WRKSRC} && \
        ${GMAKE} BINDIR=${LOCALBASE}/bin LIBDIR=${LOCALBASE}/lib \
                MANDIR=${LOCALBASE}/man \
		KERNEL_DIR=/tmp/pkgsrc/misc/linux-kernel/work.k3/linux-2.4.20/

Notice the KERNEL_DIR part. It is hard-coded so I need to adjust.

But I think it would be better to just have a post-extract target to
unextract to local WRKDIR using a distfile that it figures out from a
pkgsrc/misc/linux-kernel/Makefile.common.

   Jeremy C. Reed
   http://bsd.reedmedia.net/