cmGlobalNinjaGenerator: Optimize handling of known build outputs

Teach WriteUnknownExplicitDependencies to take ownership of the set of
WriteBuild outputs immediately since no other methods need the data.
This avoids re-inserting the whole set into another already populated
set.

Source/cmGlobalNinjaGenerator.cxx

@@ -955,6 +955,13 @@ void cmGlobalNinjaGenerator::WriteTargetAliases(std::ostream& os)
 
 void cmGlobalNinjaGenerator::WriteUnknownExplicitDependencies(std::ostream& os)
 {
+  // We need to collect the set of known build outputs.
+  // Start with those generated by WriteBuild calls.
+  // No other method needs this so we can take ownership
+  // of the set locally and throw it out when we are done.
+  std::set<std::string> knownDependencies;
+  knownDependencies.swap(this->CombinedBuildOutputs);
+
   //now write out the unknown explicit dependencies.
 
   //union the configured files, evaluations files and the CombinedBuildOutputs,
@@ -971,7 +978,6 @@ void cmGlobalNinjaGenerator::WriteUnknownExplicitDependencies(std::ostream& os)
   cmLocalNinjaGenerator *ng =
     static_cast<cmLocalNinjaGenerator *>(this->LocalGenerators[0]);
 
-  std::set<std::string> knownDependencies;
   for (std::vector<cmLocalGenerator *>::const_iterator i =
        this->LocalGenerators.begin(); i != this->LocalGenerators.end(); ++i)
     {
@@ -1026,15 +1032,6 @@ void cmGlobalNinjaGenerator::WriteUnknownExplicitDependencies(std::ostream& os)
     knownDependencies.insert( ng->ConvertToNinjaPath(i->first) );
     }
 
-  //insert outputs from all WirteBuild commands
-  //these paths have already be encoded when added to CombinedBuildOutputs
-  knownDependencies.insert(this->CombinedBuildOutputs.begin(),
-                           this->CombinedBuildOutputs.end());
-
-  //after we have combined the data into knownDependencies we have no need
-  //to keep this data around
-  this->CombinedBuildOutputs.clear();
-
   //now we difference with CombinedCustomCommandExplicitDependencies to find
   //the list of items we know nothing about.
   //We have encoded all the paths in CombinedCustomCommandExplicitDependencies