allow to switch to the integrated gpu on MacOS

without that key, the client automatically runs on the dedicated gpu (if
present). now it allows the system to use the integrated one.

closes #501

cmake/modules/MacOSXBundleInfo.plist.in

@@ -28,6 +28,8 @@
         <string>@MIRALL_VERSION_STRING@</string>
         <key>NSHumanReadableCopyright</key>
         <string>(C) 2014-2018 @APPLICATION_VENDOR@</string>
+        <key>NSSupportsAutomaticGraphicsSwitching</key>
+        <true/>
         <key>SUShowReleaseNotes</key>
         <false/>
         <key>SUPublicDSAKeyFile</key>