diff --git a/include/mbedtls/threading.h b/include/mbedtls/threading.h
index dc724ee..1b13deb 100644
--- a/include/mbedtls/threading.h
+++ b/include/mbedtls/threading.h
@@ -28,7 +28,9 @@
  * 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.
  */
+#if !defined(_POSIX_C_SOURCE)
 #define _POSIX_C_SOURCE 200112L
+#endif
 
 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "config.h"