macOS fixes

- macos_ instead of macos.  Appears to hae been missed when merging a
  previously made change in the mainline.
- Additional linker flags required for building with SDL2.  Unclear why
  pkg-config provides these flags when building with SDL1 but not with SDL2 --
  it's inconsistent but strictly not a bug, may need to discuss with the
  MacPorts team.

src/tuxpaint.c

@@ -23675,7 +23675,7 @@ static void setup_config(char *argv[])
       result = find_directory(B_USER_SETTINGS_DIRECTORY, volume, false, buffer, sizeof(buffer));
       asprintf((char **)&savedir, "%s/%s", buffer, "TuxPaint");
 #elif __APPLE__
-      savedir = strdup(macos.preferencesPath());
+      savedir = strdup(macos_preferencesPath());
 #elif __ANDROID__
       savedir = SDL_AndroidGetExternalStoragePath();
 #else