<errno.h> can't be used with gcc -traditional because it uses the "const"
keyword.  Other header files seem to solve that by using <sys/cdefs.h>, but
I don't know if that's appropriate for <errno.h>.  It's getting in the way
of compiling MH (though, amusingly enough, most of the uses of errno.h turn
out to be after some other header file has already included <sys/cdefs.h>,
but ONE file puts #include <errno.h> first...).
I've filed a PR.