enable automatic ECDH when possible (openssl 1.0.2)

Openssl 1.1.0 and later are enabling ECDH automatically, but for older
version it must be enabled explicitly or all Perfect Forward Secrecy
ciphers will be silently ignored. See also [1]. This commit applies the
same fix as found in CnetOS 7 httpd package to enable automatic ECDH as
found in [2].

[1] https://wiki.openssl.org/index.php/Diffie-Hellman_parameters
[2] https://git.centos.org/blob/rpms!httpd.git/c7/SOURCES!httpd-2.4.6-ssl-ecdh-auto.patch

common/ssl_calls.c

@@ -592,6 +592,9 @@ ssl_tls_accept(struct ssl_tls *self, long ssl_protocols,
                      SSL_MODE_ACCEPT_MOVING_WRITE_BUFFER |
                      SSL_MODE_ENABLE_PARTIAL_WRITE);
     SSL_CTX_set_options(self->ctx, options);
+#if defined(SSL_CTX_set_ecdh_auto)
+    SSL_CTX_set_ecdh_auto(self->ctx, 1);
+#endif
 
     if (g_strlen(tls_ciphers) > 1)
     {