Define H5_USER_16_API to enable HDF5 backwards compatibility

Since aiori-HDF5 uses old 1.6-style function calls (create, open, etc),
turn on the backwards compatiblity here (the one place HDF5 calls are
made).

src/aiori-HDF5.c

@@ -19,6 +19,9 @@
 #include <stdio.h>              /* only for fprintf() */
 #include <stdlib.h>
 #include <sys/stat.h>
+/* HDF5 routines here still use the old 1.6 style.  Nothing wrong with that but
+ * save users the trouble of  passing this flag through configure */
+#define H5_USE_16_API
 #include <hdf5.h>
 #include <mpi.h>