On Debian 9 (“stretch”) the `OPENSSL_NO_STATIC_ENGINE` macro is not
defined, which causes all the `#if HAVE_ENGINE_LOAD_…` directives to
fail with `error: 'HAVE_ENGINE_LOAD_…' is not defined, evaluates to 0
[-Werror,-Wundef]` while building TruffleRuby.

We can accomplish the same thing with `#ifdef`, which (of course) works
fine when the `HAVE_ENGINE_LOAD…` macros are also undefined.

Upstreamed from oracle/truffleruby#2255, which fixed
oracle/truffleruby#2254.

https://github.com/ruby/openssl/commit/65e2adf1ac