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/util/threadnames.cpp

@@ -29,7 +29,7 @@ static void SetThreadName(const char* name)
     ::prctl(PR_SET_NAME, name, 0, 0, 0);
 #elif (defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__DragonFly__))
     pthread_set_name_np(pthread_self(), name);
-#elif defined(MAC_OSX)
+#elif defined(__APPLE__)
     pthread_setname_np(name);
 #else
     // Prevent warnings for unused parameters...