Problem: ctx.cpp still uses old HAVE_LIBSODIUM

I changed this to ZMQ_USE_SODIUM to be consistent with other
configuration options (especially ZMQ_USE_TWEETNACL).

Solution: fix it.

src/ctx.cpp

@@ -99,7 +99,7 @@ zmq::ctx_t::ctx_t () :
     // allow opening of /dev/urandom
     unsigned char tmpbytes[4];
     randombytes(tmpbytes, 4);
-#elif defined (HAVE_LIBSODIUM)
+#elif defined (ZMQ_USE_SODIUM)
     int rc = sodium_init ();
     zmq_assert (rc != -1);
 #endif