Fix build with OpenSSL 3.5+ where ENGINE API is removed

[afrind]

OpenSSL removed the ENGINE API in version 3.5. The engine.h header still exists but is empty, causing implicit function declaration errors for ENGINE_by_id, ENGINE_free, etc.

Guard all ENGINE usage in the test with a PICOTLS_HAVE_ENGINE macro that checks both OPENSSL_NO_ENGINE and OPENSSL_VERSION_MAJOR. The affected code is only used for QAT hardware accelerator benchmarks and loading built-in engines on pre-3.0 OpenSSL, so no test coverage is lost on modern OpenSSL.

[kazuho]

Thank you for the pull request.

I'd be happy to add conditional guards around the use of the engine API, but I'm not sure how I can reproduce it.

The CI matrix covers OpenSSL 3.5, and engine.h still exists in OpenSSL 3.6 (https://github.com/openssl/openssl/blob/openssl-3.6/include/openssl/engine.h). Could it be due to you using a different build setting for OpenSSL? If so, I'd appreciate you describing how, as it might make sense for us to have that build setting covered in the CI matrix.

[afrind]

Could it be due to you using a different build setting for OpenSSL? If so, I'd appreciate you describing how, as it might make sense for us to have that build setting covered in the CI matrix.

This was on a CentOS stream-10 host. Maybe the system ssl was compiled with engine APIs off?

[kazuho]

Thank you! I haven't checked exactly why, but AI did reproduce the issue and provided me a Dockerfile that we can use to automate testing on CentOS Stream 10.