openssl: fix build failure on windows

Backport of: https://github.com/openssl/openssl/commit/5c5e7e



Original commit message:

    Fix build failure on Windows due to undefined cflags identifier

Reviewed-by: Tim Hudson <tjh@openssl.org>

PR-URL: https://github.com/iojs/io.js/pull/289


Reviewed-By: Fedor Indutny <fedor@indutny.com>