Philippe 22/05/2018: detect OpenACC 2.6 standard

src/LIB/SURCOUCHE/src/mode_mnh_world.f90

@@ -93,6 +93,8 @@ CONTAINS
             PRINT *,"Using OpenACC 2.0"
           ELSE IF ( OPENACC_VERSION == 201510 ) THEN
             PRINT *,"Using OpenACC 2.5"
+          ELSE IF ( OPENACC_VERSION == 201711 ) THEN
+            PRINT *,"Using OpenACC 2.6"
           ELSE
             PRINT *,"Using OpenACC (unknown version)"
           ENDIF