diff --git a/library/threading.c b/library/threading.c
index 7231f2f..c1834ba 100644
--- a/library/threading.c
+++ b/library/threading.c
@@ -23,7 +23,9 @@
  * 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 "mbedtls/config.h"