Hi I remember that we patched this to work when I tried to build mksh for plan9 a few years ago. Build.sh works perfectly fine (native compilers) even if MKSH_NOPROSPECTOFWORK is still needed for 386. I had never tried any other architectures.
Now there is a very recent attempt to make a ports system for one of the Plan9 forks (9front, although it should be possible to use on other Plan9 distros too). There it was found that the build failed on amd64: https://bitbucket.org/mveety/9front-ports/issue/13/shells-mksh-doesnt-build-on-9front I think it could be interesting to get this resolved for several archs if possibble :)
