Add More try_compile Feature Probes

[maddeleine]

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Problem:

We should attempt to turn all of our weird libcrypto version-checking logic into try_compile/feature probes. It makes our code much cleaner and easier to read. To do this we have to create a file in our tests/features folder that attempts to use a specific header or calls a specific function, and add a try_compile for that feature in both our CMakeLists.txt file and s2n.mk file.

Example of our feature probe for evp_md5_sha1: #3128

CMakeLists.txt
s2n.mk
feature probe file
Ideally we would also write a feature_probe unit test checking that if we are using a specific platform/libcrypto version/etc... this feature variable will be turned on. This lets us double check that our features are actually getting turned on.

Very incomplete list of features that probably can be try_compiled

• RSA_PSS_SIGNING_SUPPORTED
• S2N_AES_SHA256_COMPOSITE_AVAILABLE
• S2N_AES_SHA1_COMPOSITE_AVAILABLE
• S2N_GCC_VERSION_AT_LEAST(4,6,0) || defined(__clang__) for GCC diagnostic support