Add optional support for compiling against the system ssl

Controlled via some sort of optional build flag for the first iteration, while we work to get all the pieces working together.

A quick and dirty test was done in !5209 (closed)

A proper implementation should:

• have a helper like we do for include_ee? rather than having everywhere check the environment variable (so we can more easily change the env variable name, and later just change it to be based on the platform)
• The openssl depedency is skipped
• All software that currently uses the included openssl now builds with/links against the system ssl

Outside the scope of this issue:

• dealing with the trusted cert directory differences between the system and bundled openssl, will be a followup issue

Note:

That during the investigation in !5209 (closed), libgit2 and krb5 presented some anomalies that were workaround around in the test, but will need to be properly investigated and handled as part of this issue.