feature probe build environment doesnt match when building s2n-tls

[toidiu]

I was unable to resolve SOL_TCP ( #include <netinet/tcp.h>) when working on kTLS. However, when writing a feature probe for kTLS, I was able to resolve SOL_TCP.

This suggests that the feature probe is compiled with different flags than s2n-tls code. This is non-ideal since the feature probe is supposed to mimic the environment that s2n-tls is built.