hdf5_ep

[svn-r19813] 1. Correct mistake for previous checkin rev #19781:
   file name listed for DISTCLEANFILES should be testlinks_env.sh
2. Add comments to test_4() in test/external.c about the link name "/ link".
3. Fix memory leak as reported by valgrind in src/H5Lexternal.c: 
   free memory pointed to by tmp_env_prefix for HDF5_EXT_PREFIX case.