incorrect feature probes for CMake add_subdirectory builds

[jmayclin]

Problem:

The s2n-tls Cmake build includes support for an "in-source" build. Context for this support can be found in #2714 which added the functionality.

Using this in-source build will disable several important features, because s2n-tls feature probes (try-compiles) are not correctly executed when using the "in-source" build.

Solution:

Unknown.

Option 1: Remove Support For In Source Build

• add documentation explicitly warning against using add_subdirectory with s2n-tls.
• remove all in-source build functionality

Option 2: Document Limitation

For customers that always build with the same libcrypto, they might prefer a workaround that just reuses statically known libcrypto capabilities.

For example.

1. a customer runs the feature_probes out of band to generate a configuration file
2. a customer checks in the configuration file
3. a customer uses the in-source build with the configuration file. No try-compiles are executed, but features are still correctly configured
4. a customer continuously runs the feature_probes out of band (e.g. in ci) to assert on the correctness of the configuration file.

I haven't thought deeply about what s2n-tls would need to provide to enable this, but a good starting point would be documentation and a working example.