Juan 19/06/2020: add fake OPENACC module to compile with -DMNH_OPENACC...

Juan 19/06/2020: add fake OPENACC module to compile with -DMNH_OPENACC -D_FAKEOPENACC for compiler without OpenACC

src/MNH/mode_openacc.f90

+#ifdef _FAKEOPENACC
+MODULE OPENACC
+END MODULE OPENACC
+#else
+MODULE OPENACC_FAKEOPENACC
+END MODULE OPENACC_FAKEOPENACC
+#endif