CMake: Keep cmake state in a temporary directory till first build

This avoids creating lots of build directories as the user types
in something into the builddirectory line of the build settings.

Change-Id: Ib08a0f65e08bce104e4baf9e19fb01730d2f5f08
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/plugins/cmakeprojectmanager/cmakebuildconfiguration.h

@@ -75,6 +75,7 @@ public:
 
     void parse();
     void resetData();
+    bool persistCMakeState();
 
 signals:
     void errorOccured(const QString &message);