$ make regular-xfce.iso
[skipped the noise]
17:30:40 preparing distro config: build/distcfg.mk
make[1]: *** No rule to make target 'use/vmguest/vbox/dri', needed by 'use/vmguest/dri'.  Stop.
make: *** [Makefile:60: regular-xfce.iso] Error 1

Define dummy use/vmguest/vbox/dri use/vmguest/vmware/dri targets
for non-x86 architecures to avoid the problem.