[configparser.cc] Include config.h

Merged from trunk@6355.

[[Imported from SVN: r6357]]

dune/common/configparser.cc

 // -*- tab-width: 4; indent-tabs-mode: nil; c-basic-offset: 2 -*-
 // vi: set et ts=4 sw=2 sts=2:
+
+#if HAVE_CONFIG_H
+#include "config.h"
+#endif
+
 #include "configparser.hh"
 
 #include <cstdlib>