build: Replace MAC_OSX macro with existing __APPLE__

Adopting `__APPLE__` aligns our project with broader industry practices, including those in prominent projects such as the Linux kernel (and even our own code).

See: https://sourceforge.net/p/predef/wiki/OperatingSystems/#macos

src/common/args.cpp

@@ -184,7 +184,7 @@ bool ArgsManager::ParseParameters(int argc, const char* const argv[], std::strin
     for (int i = 1; i < argc; i++) {
         std::string key(argv[i]);
 
-#ifdef MAC_OSX
+#ifdef __APPLE__
         // At the first time when a user gets the "App downloaded from the
         // internet" warning, and clicks the Open button, macOS passes
         // a unique process serial number (PSN) as -psn_... command-line
@@ -741,7 +741,7 @@ fs::path GetDefaultDataDir()
         pathRet = fs::path("/");
     else
         pathRet = fs::path(pszHome);
-#ifdef MAC_OSX
+#ifdef __APPLE__
     // macOS
     return pathRet / "Library/Application Support/Bitcoin";
 #else