Pure `getifaddrs()`/`freeifaddrs()` calls do not need any type
definitions beyond those provided by `<ifaddrs.h>` itself.
The subsequent `struct ifaddrs` processing in `netif.cpp` does not
involve any symbols from `<sys/types.h>`.
We really just want to skip this when building for Windows. So do that,
and remove the two header checks (we also already use both of these
headers, unguarded, in the !windows part of the codebase).
Squash the two *iffaddrs defines into one, as I haven't seen an
iffaddrs.h that implements one, but not the other.
CMake assumes the default value internally, so overriding this causes
problems. The minimal speedup of skipping the linker isn't worth the
complexity of setting it to static.
