Target copy ctor should initialize internal state

The commit "Target copy ctor should copy internal state" created a new
cmTargetInternals constructor but failed to initialize a POD member that
the original constructor initializes.  This commit fixes it.

Source/cmTarget.cxx

@@ -57,6 +57,7 @@ public:
     }
   cmTargetInternals(cmTargetInternals const& r)
     {
+    this->SourceFileFlagsConstructed = false;
     // Only some of these entries are part of the object state.
     // Others not copied here are result caches.
     this->SourceEntries = r.SourceEntries;