I don't have the build log at hand, but a quick look at configure.ac explains the problem: - target_cpu != i386 - platform = efi if any of those two conditions is met while --enable-efiemu is passed, configure will die. Seems to be a case of REQUIRED_USE in regard of first one, but something different in second case.
I modified the logic so that --enable-efiemu will only be passed in conditions where it is valid. This is in grub-2.00_rc1. There probably should be a REQUIRED_USE for this, but it would be quite ugly and verbose.