Make as many third_party libs as possible optional

[nilsnolde]

while it's very convenient for us, it's not compatible with general package managing rules, to have public interface depending on vendored libraries. I guess all libs only used in tests are fine, no one wants to package (our) tests. this could also be (partly, see below) solved by removing those headers from our public interface which would be a great thing to do anyways.

I'll still need to confirm, but I think even compile-time header-only libs are at least frowned upon. because of security implications and possibly licensing problems.

[nilsnolde]

Scope changed a bit: removing them is too much. Making their usage optional is how I should’ve phrased it.