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.

cmake/MacOSXBundleInfo.plist.in

@@ -40,5 +40,9 @@
     <true/>
     <key>NSHumanReadableCopyright</key>
     <string>${MACOSX_BUNDLE_COPYRIGHT}</string>
+    <key>SUPublicEDKey</key>
+    <string>${MACOSX_SPARKLE_UPDATE_PUBLIC_KEY}</string>
+    <key>SUFeedURL</key>
+    <string>${MACOSX_SPARKLE_UPDATE_FEED_URL}</string>
 </dict>
 </plist>