I was curious to see how difficult it would be to compile NetBSD on FreeBSD 10. Once you have the source, there is a build.sh script which is very helpful. The build is as simple as:
./build.sh -m i386 tools ./build.sh -m i386 kernel=GENERIC release
In my case I have both clang and gcc installed. I believe the cross-compile tools were built with clang. The resulting gcc cross-compiler "i486--netbsdelf-gcc" was used to build the NetBSD source.
The generated binaries appear in obj/releasedir/i386/binary/