In order to separate OpenSSL's SHA symbols, this header has been using
USE_SSL, which is equivalent to USE_OPENSSL.  There is now only one SSL
implementation included in the tree, so this works fine, but when
adding a new SSL implementation this would run into failures.

Author: Daniel Gustafsson
Discussion: https://postgr.es/m/0DF29010-CE26-4F51-85A6-9C8ABF5536F9@yesql.se