Backwards support for OpenSSL 1.0.x broken

[gdanezis]

Ok, so we have now upgraded to support OpenSSL 1.1 API, but in the process the support for OpenSSL 1.0 was broken. Have a look at the failing Travis test, and despair. This is likely to require a conditional compilation directive to hide the bignum_st structure redefinition and also a conditional compilation directive to use old-style locking for the G2_mul pre-computations.

Failing travis tests: https://travis-ci.org/gdanezis/bplib