[macOS] Build infrastructure and portability fixes

Build system:
- Enable ObjC++, macOS deployment target, framework linking
- Fix AppleClang detection
- macOS platform name for output directories
- Skip Vulkan on macOS, null graphics without Vulkan dependency
- macOS SDL2 framework detection and discord-rpc macOS sources

Portability fixes:
- mach_absolute_time for macOS tick counting
- _NSGetExecutablePath for macOS
- guard MAP_FIXED_NOREPLACE, ftruncate64->ftruncate
- guard Vulkan include/usage, fix data root for all POSIX

third_party/CMakeLists.txt

@@ -121,6 +121,12 @@ if(WIN32)
     discord-rpc/src/connection_win.cpp
     discord-rpc/src/discord_register_win.cpp
   )
+elseif(APPLE)
+  target_sources(discord-rpc PRIVATE
+    discord-rpc/src/connection_unix.cpp
+    discord-rpc/src/discord_register_osx.m
+  )
+  target_link_libraries(discord-rpc PRIVATE "-framework AppKit")
 elseif(CMAKE_SYSTEM_NAME STREQUAL "Linux")
   target_sources(discord-rpc PRIVATE
     discord-rpc/src/connection_unix.cpp