[cmake] refactor: SDL2 CI and CPMUtil::AddCIPackage (#284)

Replaces bundled SDL2 with my SDL2 CI, and updates external SDL2

Additionally, reduces all that boilerplate with a common AddCIPackage in
CPMUtil.cmake, currently used by OpenSSL and SDL2. To be used with ffmpeg in the future

Signed-off-by: crueter <crueter@crueter.xyz>
Reviewed-on: https://git.eden-emu.dev/eden-emu/eden/pulls/284
Reviewed-by: Lizzie <lizzie@eden-emu.dev>
Reviewed-by: Shinmegumi <shinmegumi@eden-emu.dev>

src/core/internal_network/sockets.h

@@ -10,7 +10,7 @@
 
 #if defined(_WIN32)
 // windows
-#elif defined(__unix__)
+#elif defined(__unix__) || defined(__APPLE__)
 // unix
 #else
 // haiku
@@ -28,7 +28,7 @@ struct ProxyPacket;
 
 class SocketBase {
 public:
-#ifdef __unix__
+#if defined(__unix__) || defined(__APPLE__)
     using SOCKET = int;
     static constexpr SOCKET INVALID_SOCKET = -1;
     static constexpr SOCKET SOCKET_ERROR = -1;