build: simplify *ifaddr handling

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/module/TestAppendRequiredLibraries.cmake

@@ -38,8 +38,7 @@ function(test_append_socket_library target)
       message(FATAL_ERROR "Cannot figure out how to use getifaddrs/freeifaddrs.")
     endif()
   endif()
-  set(HAVE_DECL_GETIFADDRS TRUE PARENT_SCOPE)
-  set(HAVE_DECL_FREEIFADDRS TRUE PARENT_SCOPE)
+  set(HAVE_IFADDRS TRUE PARENT_SCOPE)
 endfunction()
 
 # Clang, when building for 32-bit,