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)
