Add missing _POSIX_C_SOURCE define in threading.h
#ifndef MBEDTLS_THREADING_H
#define MBEDTLS_THREADING_H
+/*
+ * 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 _POSIX_C_SOURCE 200112L
+
#if !defined(MBEDTLS_CONFIG_FILE)
#include "config.h"
#else
