fvwm-2.5.16 is marked as depending on debianutils, which makes it unavailable on Gentoo/*BSD archs. There does not seem to be any reason for fvwm to depend on that, as it builds apparently well without it.
Also pretty much pointless, considering that debianutils is in base profile's packages.
It requires mktemp shell command, is that provided on bsd?
Yes, just use userland_GNU? ( debianutils ) and you'll be set.
Ive added that condition.