Conditionally use input file

If config.h is written, then the build system will also define
HAVE_CONFIG_H.

opm/core/io/eclipse/BlackoilEclipseOutputWriter.cpp

@@ -18,7 +18,10 @@
   You should have received a copy of the GNU General Public License
   along with OPM.  If not, see <http://www.gnu.org/licenses/>.
  */
+#if HAVE_CONFIG_H
 #include "config.h"
+#endif // HAVE_CONFIG_H
+
 #include "BlackoilEclipseOutputWriter.hpp"
 
 #include <opm/core/grid.h>