ProjectExplorer: Make BuildSystem:Name variable available

... for use in the default build dir template.

Task-number: QTCREATORBUG-26147
Change-Id: I1a32d60e0d5e2db514ac315c48c615c55ccda51f
Reviewed-by: hjk <hjk@qt.io>

src/plugins/projectexplorer/buildconfiguration.h

@@ -115,7 +115,8 @@ public:
                                                       const QString &projectName,
                                                       const Kit *kit,
                                                       const QString &bcName,
-                                                      BuildType buildType);
+                                                      BuildType buildType,
+                                                      const QString &buildSystem);
 
     bool isActive() const;