diff --git a/crypto/rand/urandom.c b/crypto/rand/urandom.c
index 2572625..f0f31f2 100644
--- a/crypto/rand/urandom.c
+++ b/crypto/rand/urandom.c
@@ -12,7 +12,9 @@
  * OF CONTRACT, NEGLIGENCE OR OTHER TORTIOUS ACTION, ARISING OUT OF OR IN
  * CONNECTION WITH THE USE OR PERFORMANCE OF THIS SOFTWARE. */
 
+#if !defined(_GNU_SOURCE)
 #define _GNU_SOURCE  /* needed for syscall() on Linux. */
+#endif
 
 #include <openssl/rand.h>