Add Swift Package Manager support

[colemancda]

For macOS, iOS and tvOS compilation

[nekonomicon]

1. Don't include all files.
   Use file lists from waf/cmake scripts.
2. Don't use LINUX or similar macro.
   As an example mp_stubs.cpp does not contain windows specific code, but this file contains stubs for multiplayer-only mods.
3. It would be good to use waf instead of any alternative build system.
4. Drop of OSX/iOS support xash3d-fwgs#61

[a1batross]

I doubt we would add something than Waf, CMake and Android.mk. Even Android.mk will deprecated someday.