Add OpenSSL 4.x support and switch to provider-only mode

[olszomal]

Pull Request Type

• Bug fix
• New feature
• Code style / formatting / renaming
• Refactoring (no functional or API changes)
• Build / CI related changes
• Documentation
• Other (please describe): Enable OpenSSL 4.x support

Related Issue

Issue number: #631

Current Behavior

libp11 does not build correctly with OpenSSL 4.x.

New Behavior

libp11 builds cleanly with OpenSSL 4.x

Scope of Changes

• Detect OpenSSL 4.x and enable provider mode in the build system.
• Guard ENGINE-related code with OPENSSL_NO_ENGINE.
• Disable legacy custom EVP_PKEY_METHOD handling for OpenSSL 4.x.
• Adjust or skip ENGINE-based tests when running against OpenSSL 4.x.

Testing

• Existing tests
• New tests added
• Manual testing
  Tested with OpenSSL 3.x (all tests passing).
  Some OpenSSL 4.0.0-dev test failures are expected at this stage and will be handled in subsequent PRs.

Additional Notes

Behavior for OpenSSL ≤ 3.x is preserved.
In OpenSSL 4.x, the OPENSSL_NO_ENGINE macro is always defined. ENGINE-based code paths are therefore intentionally disabled.

License Declaration

• I hereby agree to license my contribution under the project's license.

[mtrojnar]

I think returning 0 (success) in tests that are never supposed to be executed could be confusing. Should we return something else, such as 1 or 77, just in case someone decides to execute this code anyway? Printing an error message would be a good idea as well.