h5check

Fix bug #1970 when the user used hdf5-1.8.2 h5cc to compile test/testgen.c in
h5check.  The problem was due to H5Z_class_t v.s. H5Z_class2_t.  
Both structures were the same in 1.8*.  
It was named H5Z_class2_t in >= 1.8.4 releases but was called
H5Z_class_t in 1.8* but < 1.8.4.