hdf5_ep

[svn-r19824] 1. Made fixes as in rev #19813 in trunk:
   a) Add comments to test_4() in test/external.c about the name "/ link".
   b) Fix one memory leak in src/H5Lexternal.c: free memory pointed to by tmp_env_prefix for HDF5_EXT_PREFIX case.
3. Made changes for bug #1930 as in rev #19783, #19781 in trunk.
4. Made corresponding CMake changes as in rev #19782 in trunk.