thread-safe crypto: cast thread identifiers to pointers

Use pointer-based thread identifiers instead of number-based ones. This
should resolve compilation warnings on BSD systems.

Linux uses integers as thread identifiers, BSD based systems use pointers.

ref #157

src/libknot/dnssec/crypto.c

@@ -104,7 +104,7 @@ static void openssl_mutexes_destroy(void)
 static void openssl_threadid_cb(CRYPTO_THREADID *openssl_id)
 {
 	pthread_t id = pthread_self();
-	CRYPTO_THREADID_set_numeric(openssl_id, id);
+	CRYPTO_THREADID_set_pointer(openssl_id, (void *)id);
 }
 
 /*- public API --------------------------------------------------------------*/