Enable snprintf on FreeBSD
/* Ensure gmtime_r is available even with -std=c99; must be included before
* config.h, which pulls in glibc's features.h. Harmless on other platforms. */
-#define _XOPEN_SOURCE 500
+#define _POSIX_C_SOURCE 200112L
#if !defined(MBEDTLS_CONFIG_FILE)
#include "mbedtls/config.h"
