CMake: Cache buildtargets in builddirmanager

No need to have the builddirreaders to so.

Change-Id: Ia7e38f6483b29d6a4f4b65baacc17bf184a3cb34
Task-number: QTCREATORBUG-17359
Reviewed-by: Florian Apolloner <florian@apolloner.eu>
Reviewed-by: Tobias Hunger <tobias.hunger@qt.io>

src/plugins/cmakeprojectmanager/builddirmanager.h

@@ -114,6 +114,8 @@ private:
     std::unique_ptr<BuildDirReader> m_reader;
     QFutureWatcher<QList<ProjectExplorer::FileNode*>> m_futureWatcher;
     QFuture<QList<ProjectExplorer::FileNode*>> m_scanFuture;
+
+    mutable QList<CMakeBuildTarget> m_buildTargets;
 };
 
 } // namespace Internal