fix: Add implicit gcc flag to all feature probes

[goatgoose]

Description of changes:

A recent refactor of our feature probes in #3921 correctly caused the rust bindings to probe for more features. However, due to how some of the feature probes work, building s2n-tls-sys currently fails when linked with some libcryptos.

In CMake, a feature is enabled after successfully compiling the feature test and linking it with the libcrypto. Some feature probes, such as S2N_LIBCRYPTO_SUPPORTS_EVP_MD5_SHA1_HASH, rely on the linker failing to disable the feature:

CMakeFiles/cmTC_e26ac.dir/S2N_LIBCRYPTO_SUPPORTS_EVP_MD5_SHA1_HASH.c.o: In function `main':
S2N_LIBCRYPTO_SUPPORTS_EVP_MD5_SHA1_HASH.c:(.text+0x8): undefined reference to `EVP_md5_sha1'
collect2: error: ld returned 1 exit status

In the rust bindings, the feature probe is compiled but not linked with the libcrypto. This causes the compilation to produce a warning, but the feature probe is still enabled:

[s2n-tls-sys 0.0.32] === Testing feature S2N_LIBCRYPTO_SUPPORTS_EVP_MD5_SHA1_HASH ===
[s2n-tls-sys 0.0.32] lib/tests/features/S2N_LIBCRYPTO_SUPPORTS_EVP_MD5_SHA1_HASH.c: In function ‘main’:
[s2n-tls-sys 0.0.32] lib/tests/features/S2N_LIBCRYPTO_SUPPORTS_EVP_MD5_SHA1_HASH.c:18:5: warning: implicit declaration of function ‘EVP_md5_sha1’; did you mean ‘LN_md5_sha1’? [-Wimplicit-function-declaration]
[s2n-tls-sys 0.0.32]      EVP_md5_sha1();
[s2n-tls-sys 0.0.32]      ^~~~~~~~~~~~
[s2n-tls-sys 0.0.32]      LN_md5_sha1
[s2n-tls-sys 0.0.32] S2N_LIBCRYPTO_SUPPORTS_EVP_MD5_SHA1_HASH: true

To fix this, the -Werror=implicit-function-declaration gcc flag was added, causing the compiler to error when a function isn't declared without relying on the linker to fail. Since this kind of check is common with feature probing, this flag was added to a new GLOBAL.flags file which gets applied to all of the feature probes.

Call-outs:

• Ideally, the rust feature probe implementation would match the c implementation, and link to the libcrypto after compiling each feature test. However, I wasn't sure how to obtain the path to the libcrypto library artifact from openssl-sys to pass to the compiler command, so I left this change out of this PR. It seems like we're able to get the header path from openssl-sys with DEP_OPENSSL_INCLUDE, but I couldn't find an equivalent environment variable for the library directory.

Testing:

This change is tested with a new OpenSSL 1.0.2 generate job added in this PR. OpenSSL 1.0.2 does not define EVP_md5_sha1, so the S2N_LIBCRYPTO_SUPPORTS_EVP_MD5_SHA1_HASH feature should not be enabled. Without this change, S2N_LIBCRYPTO_SUPPORTS_EVP_MD5_SHA1_HASH is incorrectly enabled and this CI job fails:

warning: lib/crypto/s2n_hash.c:60:20: warning: implicit declaration of function ‘EVP_md5_sha1’; did you mean ‘NID_md5_sha1’? [-Wimplicit-function-declaration]
warning:    60 |             return EVP_md5_sha1();
warning:       |                    ^~~~~~~~~~~~
warning:       |                    NID_md5_sha1
warning: lib/crypto/s2n_hash.c:60:20: warning: returning ‘int’ from a function with return type ‘const EVP_MD *’ {aka ‘const struct env_md_st *’} makes pointer from integer without a cast [-Wint-conversion]
warning:    60 |             return EVP_md5_sha1();
warning:       |                    ^~~~~~~~~~~~~~
   Compiling s2n-tls v0.0.32 (/home/runner/work/s2n-tls/s2n-tls/bindings/rust/s2n-tls)
error: linking with `cc` failed: exit status: 1

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.