Allow for configuring OpenSSL

Let OpenSSL load its configuration file.  See the OPENSSL_config(3) man
page for details.

(Suggested by Max Kosmach in Debian bug report #689960, forwarded by Jan
Wagner.)

plugins/common.h

@@ -141,6 +141,7 @@
 #ifdef HAVE_GNUTLS_OPENSSL_H
 #  include <gnutls/openssl.h>
 #else
+#  define OPENSSL_LOAD_CONF /* See the OPENSSL_config(3) man page. */
 #  ifdef HAVE_SSL_H
 #    include <rsa.h>
 #    include <crypto.h>