Skip anon cipher test if constant is unavailable, as with OpenSSL < 1.2

We define this constant on the ExtConfig module at compile time,
based on the cipher lists of OpenSSL:
  https://www.openssl.org/docs/manmaster/apps/ciphers.html

This is because the `start_server` test helper method defaults to use this
cipher for anonymous connections. Currently:

```ruby
def start_server(verify_mode, start_immediately, args = {}, &block)
  # ...
  use_anon_cipher = args.fetch(:use_anon_cipher, false)
  ctx.ciphers = "ADH-AES256-GCM-SHA384" if use_anon_cipher
  # ...
end
```

This _should_ fix the build. Patch reviewed by @nobu /cc @hsbt

1 files changed, 6 insertions, 0 deletions

diff --git a/ext/openssl/ossl_ssl.c b/ext/openssl/ossl_ssl.c
index 09d8dd24..538ba37f 100644
--- a/ext/openssl/ossl_ssl.c
+++ b/ext/openssl/ossl_ssl.c
@@ -2107,6 +2107,12 @@ Init_ossl_ssl(void)
     rb_define_const(mSSLExtConfig, "HAVE_TLSEXT_HOST_NAME", Qfalse);
 #endif
 
+#ifdef TLS_DH_anon_WITH_AES_256_GCM_SHA384
+    rb_define_const(mSSLExtConfig, "TLS_DH_anon_WITH_AES_256_GCM_SHA384", Qtrue);
+#else
+    rb_define_const(mSSLExtConfig, "TLS_DH_anon_WITH_AES_256_GCM_SHA384", Qfalse);
+#endif
+
     /*
      * A callback invoked whenever a new handshake is initiated. May be used
      * to disable renegotiation entirely.