make config.h the first header to be included in any compile unit

this is required for consistency amongst the compile units which are
linked into the same library and seems to be forgotten quite
frequently.

examples/wells_example.cpp

@@ -1,3 +1,4 @@
+#include "config.h"
 #include <iostream>
 #include <opm/core/io/eclipse/EclipseGridParser.hpp>
 #include <opm/core/utility/parameters/ParameterGroup.hpp>