Improve detection for DES support in OpenSSL, to allow compilation
against system OpenSSL without DES.

Note that MariaDB needs to be compiled against OpenSSL-like library
that itself has DES support which cmake detected. Positive detection
is indicated with CMake variable HAVE_des 1.
Signed-off-by: Dimitri John Ledkov <dimitri.ledkov@surgut.co.uk>