[asio] Ignore SSLv2 when using BoringSSL

BoringSSL doesn't allow the use of (the deeply-flawed) SSL version 2. It
also doesn't allow consumers to reach directly inside the SSL struct,
which is what asio was using to detect SSLv2 for OpenSSL pre-1.0.1.
This CL simply extends the preprocessor conditional to include
BoringSSL.

Change-Id: Ie423f2b9f1394fca83da9b0b23f176bac59a4c03
diff --git a/asio/include/asio/ssl/detail/impl/engine.ipp b/asio/include/asio/ssl/detail/impl/engine.ipp
index 5566149..4133f26 100644
--- a/asio/include/asio/ssl/detail/impl/engine.ipp
+++ b/asio/include/asio/ssl/detail/impl/engine.ipp
@@ -203,10 +203,10 @@
 
   // SSL v2 doesn't provide a protocol-level shutdown, so an eof on the
   // underlying transport is passed through.
-#if (OPENSSL_VERSION_NUMBER < 0x10100000L)
+#if !defined(OPENSSL_IS_BORINGSSL) && (OPENSSL_VERSION_NUMBER < 0x10100000L)
   if (ssl_->version == SSL2_VERSION)
     return ec;
-#endif // (OPENSSL_VERSION_NUMBER < 0x10100000L)
+#endif // !defined(OPENSSL_IS_BORINGSSL)&&(OPENSSL_VERSION_NUMBER < 0x10100000L)
 
   // Otherwise, the peer should have negotiated a proper shutdown.
   if ((::SSL_get_shutdown(ssl_) & SSL_RECEIVED_SHUTDOWN) == 0)