Made all libcrypto HMAC APIs use dynamic discovery

vtable all the HMAC things (#53)

* Make openssl vtables mutually exclusive

* Switched to single vtable for libcrypto, vtable'd full HMAC API

* Resolve libcrypto manually, if possible, to disambiguate when multiple .so's are loaded

* Only probe for .so libs when built as a shared lib

* aws-c-auth added as a downstream