build: fix --without-ssl compile time error

Fix the following build error by putting #if guards around the
variables:

    ../src/node.cc: In function 'void node::ParseArgs(int*,
    const char**, int*, const char***, int*, const char***)':
    ../src/node.cc:3037:7: error: 'SSL2_ENABLE' was not declared
    in this scope
           SSL2_ENABLE = true;
           ^
    ../src/node.cc:3039:7: error: 'SSL3_ENABLE' was not declared
    in this scope
           SSL3_ENABLE = true;

Fixes: https://github.com/nodejs/node-v0.x-archive/issues/8645
PR-URL: https://github.com/nodejs/node/pull/3825


Reviewed-By: Colin Ihrig <cjihrig@gmail.com>
Reviewed-By: Johan Bergström <bugs@bergstroem.nu>
Reviewed-By: James M Snell <jasnell@gmail.com>