libgcrypt: Update 1.5.3 and 1.6.2.

Also changed the description of 1.6.nix to match that of
default.nix (1.5).

* https://lists.gnu.org/archive/html/info-gnu/2014-08/msg00001.html
* https://lists.gnu.org/archive/html/info-gnu/2014-08/msg00011.html