$NetBSD$

Fix build on NetBSD.

--- Makefile.orig	2025-10-07 18:36:23.686122088 +0000
+++ Makefile
@@ -1,5 +1,5 @@
 CC = gcc
-CFLAGS = -Wall -Wextra -std=c11 -D_XOPEN_SOURCE=500
+CFLAGS = -Wall -Wextra -std=c11
 LDFLAGS =
 LIBS = -lcrypto
 PREFIX ?= /usr/local
