Starting with the v3 of CGNS/MLL, two file formats are now managed by the library: ADF and HDF5. If you build the CGNS/MLL library (that is the libcgns.a) with the default parameters, you will obtain a library with capabilities to handle these two file formats. The library tries to detect the format by scanning the actual file.
The shell variables HDF5_LINK_PATH or ADF_LINK_PATH are used to find the linked-to files. The syntax of these variables is the same as the usual PATH or LD_LIBRAR_PATH of your Unix shell.