Made PHP extension compile with openssl by default

[justinboswell]

Depends on:

• https://github.com/awslabs/aws-crt-ffi/pull/28
• https://github.com/awslabs/aws-crt-ffi/pull/35
• https://github.com/aws/s2n-tls/pull/2997
• https://github.com/aws/s2n-tls/pull/2993
• https://github.com/aws/s2n-tls/pull/3011

Also disables attempting to build FFI, which will be removed in a follow-up PR.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.