Since it wasn't possible to have specific manifest entries for !Darwin
!WINNT !Android, a few places in the tree use the following pattern:
entry-for-unix
entry-for-osx os=Darwin
entry-for-windows os=WINNT
This works because subsequent manifest entries with more specific flags
override previous manifest entries.
Incidentally, this led to problems such as the one mentioned in
changeset c94e87a18096.
Now that there is a flag for !Darwin !WINNT !Android, we can use it
instead.
2f71018f6a