Add build options for Sparkle updater

Two new build options are added:

`MAC_SPARKLE_PUB_KEY`: the public key used to verify the signatures of the appcast
`MAC_SPARKLE_APPCAST_URL`: the URL where the `appcast.xml` is located

If the updater should be disabled on macOS, set either of these to an empty string.

buildconfig/BuildConfig.h

@@ -74,6 +74,12 @@ class Config {
     /// URL for the updater's channel
     QString UPDATER_BASE;
 
+    /// The public key used to sign releases for the Sparkle updater appcast
+    QString MAC_SPARKLE_PUB_KEY;
+
+    /// URL for the Sparkle updater's appcast
+    QString MAC_SPARKLE_APPCAST_URL;
+
     /// User-Agent to use.
     QString USER_AGENT;