This was removed in e29b0da9c7, because
it was felt it was ambiguous whether isBSD should remove Darwin.
I think it should be reintroduced. Packages sometimes have their own
concepts of "is BSD" e.g. Lua, and these almost never include Darwin,
so let's keep Darwin excluded.
Without a way to say "is this BSD", one has to list all flavours of
BSD seperately, even though fundamentally they're still extremely
similar. I don't want to have to write the following!
stdenv.isFreeBSD || stdenv.isNetBSD || stdenv.isOpenBSD || stdenv.isDragonFlyBSD
Additionally, we've had stdenv.hostPlatform.isBSD this whole time, and
it hasn't hurt anything.
730a9a04fa