Print out literature references when Libxc is used

[susilehtola]

If Libxc is used, CPMD should print out a message stating that Libxc is used, along with the version of the library, provided by the xc_version_string() function.

In addition, the program should print out the citation information xc_refererence() and xc_reference_doi(), as well as the the literature references for the employed functional(s) which are also provided by Libxc.