SSL: guard use of all SSL options for bug workarounds.

Some of the OpenSSL forks (read: BoringSSL) started removing unused,
no longer necessary and/or not really working bug workarounds along
with the SSL options and defines for them.

Instead of fixing nginx build after each removal, be proactive
and guard use of all SSL options for bug workarounds.

Signed-off-by: Piotr Sikora <piotr@cloudflare.com>

src/event/ngx_event_openssl.c

diff --git a/src/event/ngx_event_openssl.c b/src/event/ngx_event_openssl.c
index bb82143..fa7cc15 100644
--- a/src/event/ngx_event_openssl.c
+++ b/src/event/ngx_event_openssl.c
@@ -206,13 +206,23 @@
 
     /* client side options */
 
+#ifdef SSL_OP_MICROSOFT_SESS_ID_BUG
     SSL_CTX_set_options(ssl->ctx, SSL_OP_MICROSOFT_SESS_ID_BUG);
+#endif
+
+#ifdef SSL_OP_NETSCAPE_CHALLENGE_BUG
     SSL_CTX_set_options(ssl->ctx, SSL_OP_NETSCAPE_CHALLENGE_BUG);
+#endif
 
     /* server side options */
 
+#ifdef SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG
     SSL_CTX_set_options(ssl->ctx, SSL_OP_SSLREF2_REUSE_CERT_TYPE_BUG);
+#endif
+
+#ifdef SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER
     SSL_CTX_set_options(ssl->ctx, SSL_OP_MICROSOFT_BIG_SSLV3_BUFFER);
+#endif
 
 #ifdef SSL_OP_MSIE_SSLV2_RSA_PADDING
     /* this option allow a potential SSL 2.0 rollback (CAN-2005-2969) */
@@ -223,10 +233,17 @@
     SSL_CTX_set_options(ssl->ctx, SSL_OP_SSLEAY_080_CLIENT_DH_BUG);
 #endif
 
+#ifdef SSL_OP_TLS_D5_BUG
     SSL_CTX_set_options(ssl->ctx, SSL_OP_TLS_D5_BUG);
-    SSL_CTX_set_options(ssl->ctx, SSL_OP_TLS_BLOCK_PADDING_BUG);
+#endif
 
+#ifdef SSL_OP_TLS_BLOCK_PADDING_BUG
+    SSL_CTX_set_options(ssl->ctx, SSL_OP_TLS_BLOCK_PADDING_BUG);
+#endif
+
+#ifdef SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS
     SSL_CTX_set_options(ssl->ctx, SSL_OP_DONT_INSERT_EMPTY_FRAGMENTS);
+#endif
 
     SSL_CTX_set_options(ssl->ctx, SSL_OP_SINGLE_DH_USE);