Don't put QString in static attributes

It crashed in some places on our code-base due to the so-called "static
initialization order fiasco".
As a solution, it turns the variable into a function. This shouldn't have
a penalty due to QStringLiteral.

Change-Id: I9f8a955afdff878dc2f0db16fec861d81250c243
Reviewed-by: Tobias Hunger <tobias.hunger@theqtcompany.com>

src/libs/cplusplus/pp-engine.h

@@ -77,7 +77,7 @@ class CPLUSPLUS_EXPORT Preprocessor
     typedef Internal::Value Value;
 
 public:
-    static const QString configurationFileName;
+    static QString configurationFileName();
 
 public:
     Preprocessor(Client *client, Environment *env);