Add build support for iOS

[horusxnetworks]

• route.h is not found on later release of iOS SDK.
• route.h is copied from MacOS SDK (from XCode 11.3.1) This commit is to support build for iOS

[Coeur]

Possibly more portable or future-proof:

#if __has_include(<net/route.h>)
#include <net/route.h>
#else
#include "route.h"
#endif

and then we don't need <TargetConditionals.h>.

[Coeur]

In Xcode 15.3 (about 4 years after 11.3.1),

• lines 86-87 (rmx_state and rmx_filler[3]) got combined into rmx_filler[4].
• line 109 (RTF_LLDATA) is removed.
• line 130 adds #define RTF_GLOBAL 0x40000000
• line 133 (RTF_BITS) is removed.
• line 142 adds macros IS_DYNAMIC_DIRECT_HOSTROUTE and IS_LOCALNET_ROUTE

The rest matches.