diff --git a/library/entropy_poll.c b/library/entropy_poll.c
index 4556f88..5248a25 100644
--- a/library/entropy_poll.c
+++ b/library/entropy_poll.c
@@ -21,8 +21,10 @@

 #if defined(__linux__)
 /* Ensure that syscall() is available even when compiling with -std=c99 */
+#if !defined(_GNU_SOURCE)
 #define _GNU_SOURCE
 #endif
+#endif

 #if !defined(MBEDTLS_CONFIG_FILE)
 #include "mbedtls/config.h"