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/fs_helpers.cpp

@@ -117,7 +117,7 @@ bool FileCommit(FILE* file)
         LogPrintf("FlushFileBuffers failed: %s\n", Win32ErrorString(GetLastError()));
         return false;
     }
-#elif defined(MAC_OSX) && defined(F_FULLFSYNC)
+#elif defined(__APPLE__) && defined(F_FULLFSYNC)
     if (fcntl(fileno(file), F_FULLFSYNC, 0) == -1) { // Manpage says "value other than -1" is returned on success
         LogPrintf("fcntl F_FULLFSYNC failed: %s\n", SysErrorString(errno));
         return false;
@@ -195,7 +195,7 @@ void AllocateFileRange(FILE* file, unsigned int offset, unsigned int length)
     nFileSize.u.HighPart = nEndPos >> 32;
     SetFilePointerEx(hFile, nFileSize, 0, FILE_BEGIN);
     SetEndOfFile(hFile);
-#elif defined(MAC_OSX)
+#elif defined(__APPLE__)
     // OSX specific version
     // NOTE: Contrary to other OS versions, the OSX version assumes that
     // NOTE: offset is the size of the file.

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...