python-m2crypto: don't reference SSLv2 functions if openssl is built without SSLv2 support

Fixes:
http://autobuild.buildroot.net/results/018/0183ba8c9fccc87f9e72279c49c2fdc1a9fcb556/



The recent openssl security bump disabled SSLv2 support, but python-m2crypto
was still referencing SSLv2 functions causing undefined symbols when the
module was imported.  Backport an upstream patch to only reference these
symbols if openssl is built with SSLv2 support.

Signed-off-by: Peter Korsgaard <peter@korsgaard.com>