Try to find XQuartz via pkgutil if mdls fails

c.f. Homebrew/homebrew#16296.

Library/Homebrew/macos/xquartz.rb

@@ -1,6 +1,7 @@
 module MacOS::XQuartz extend self
   FORGE_BUNDLE_ID = "org.macosforge.xquartz.X11"
   APPLE_BUNDLE_ID = "org.x.X11"
+  FORGE_PKG_ID = "org.macosforge.xquartz.pkg"
 
   # This returns the version number of XQuartz, not of the upstream X.org.
   # The X11.app distributed by Apple is also XQuartz, and therefore covered
@@ -10,6 +11,9 @@ module MacOS::XQuartz extend self
       path = MacOS.app_with_bundle_id(FORGE_BUNDLE_ID) || MacOS.app_with_bundle_id(APPLE_BUNDLE_ID)
       if not path.nil? and path.exist?
         `mdls -raw -name kMDItemVersion "#{path}" 2>/dev/null`.strip
+      else
+        # Some users disable Spotlight indexing. Try to find it via pkgutil
+        MacOS.pkgutil_info(FORGE_PKG_ID) =~ /version: (\d\.\d\.\d).+$/ and $1
       end
     end
   end