cmGeneratorTarget: Don't add computed sources to the target.

When config-specifig generator expressions are supported, a target
may have SOURCES:

 src1.cpp $<$<CONFIG:Debug>:src2.cpp> $<$<CONFIG:Release>:src3.cpp>

and computation in cmTargetTraceDependencies would add each of the
src2.cpp and src3.cpp sources back to the target without a
config-guard.  That would make the sources be used later when
generating the buildsystem, regardless of the configuration.

Avoid calling AddSource on the target with the result of the
GetSourceFiles call.

Source/cmGeneratorTarget.cxx

@@ -619,7 +619,11 @@ cmTargetTraceDependencies
     for(std::vector<std::string>::const_iterator si = sources.begin();
         si != sources.end(); ++si)
       {
-      this->QueueSource(*si);
+      if(this->SourcesQueued.insert(*si).second)
+        {
+        this->SourceQueue.push(*si);
+        this->Makefile->GetOrCreateSource(*si);
+        }
       }
     }