Meson: Emit parsing finished when done

Also, trigger build button re-check the hard way

This should not really be necessary, but the code paths on the
core side are still quite twisted. The extra notification here
is harmless in price, and removes the showstopper of not getting
a buildbutton.

Change-Id: If4bec892b4ebea533a95033629df60fa564985f3
Reviewed-by: Alessandro Portale <alessandro.portale@qt.io>

src/plugins/mesonprojectmanager/mesonbuildsystem.cpp

@@ -87,6 +87,9 @@ void MesonBuildSystem::parsingCompleted(bool success)
         UNLOCK(false);
         emitBuildSystemUpdated();
     }
+    emitParsingFinished(success);
+
+    emit buildConfiguration()->enabledChanged(); // HACK. Should not be needed.
 }
 
 ProjectExplorer::Kit *MesonBuildSystem::MesonBuildSystem::kit()