Protected the new (not yet finished) code that merge environments.

src/libs/cplusplus/FastPreprocessor.cpp

@@ -38,12 +38,14 @@ FastPreprocessor::FastPreprocessor(const Snapshot &snapshot)
 
 QByteArray FastPreprocessor::run(QString fileName, const QString &source)
 {
+#ifdef QTCREATOR_WITH_MERGED_ENVIRONMENT
     if (Document::Ptr doc = _snapshot.value(fileName)) {
         _merged.insert(fileName);
 
         foreach (const Document::Include &i, doc->includes())
             mergeEnvironment(i.fileName());
     }
+#endif
 
     const QByteArray preprocessed = _preproc(fileName, source);
     return preprocessed;