Fix GnuTLS 2.x compatiblity

Closes GH-216

src/LibgnutlsTLSSession.cc

@@ -75,6 +75,7 @@ GnuTLSSession::~GnuTLSSession()
 
 int GnuTLSSession::init(sock_t sockfd)
 {
+#if GNUTLS_VERSION_NUMBER >= 0x030000
   unsigned int flags = tlsContext_->getSide() == TLS_CLIENT ?
     GNUTLS_CLIENT : GNUTLS_SERVER;
 #ifdef A2_DISABLE_OCSP
@@ -84,6 +85,11 @@ int GnuTLSSession::init(sock_t sockfd)
 #endif // A2_DISABLE_OCSP
 
   rv_ = gnutls_init(&sslSession_, flags);
+#else // GNUTLS_VERSION_NUMBER >= 0x030000
+  rv_ = gnutls_init(
+      &sslSession_,
+      tlsContext_->getSide() == TLS_CLIENT ? GNUTLS_CLIENT : GNUTLS_SERVER);
+#endif // GNUTLS_VERSION_NUMBER >= 0x030000
   if(rv_ != GNUTLS_E_SUCCESS) {
     return TLS_ERR_ERROR;
   }