Fix redefinition error under OpenSSL
Fix redefinition error under OpenSSL

LIBRESSL_VERSION_NUMBER is 0 when not defined, making the condition always true.

Close #1688

znc/znc,znc/znc,DarthGandalf/znc,DarthGandalf/znc,znc/znc,znc/znc,DarthGandalf/znc,znc/znc,DarthGandalf/znc,znc/znc,DarthGandalf/znc,DarthGandalf/znc