post-review fixes:
* move all ssl implementation related ifdefs/defines to one file
  (ssl_compat.h)
* work around OpenSSL-1.1 desire to malloc every EVP context by
  run-time checking that context allocated on the stack is big enough
  (openssl.c)
* use newer version of the AWS SDK for OpenSSL 1.1
* use get_dh2048() function as generated by openssl 1.1
  (viosslfactories.c)