Fix the build when _GNU_SOURCE is defined to a non-empty value
Fix #3432.
Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>
diff --git a/library/entropy_poll.c b/library/entropy_poll.c
index 4bf660e..5250a7b 100644
--- a/library/entropy_poll.c
+++ b/library/entropy_poll.c
@@ -17,7 +17,7 @@
* limitations under the License.
*/
-#if defined(__linux__)
+#if defined(__linux__) && !defined(_GNU_SOURCE)
/* Ensure that syscall() is available even when compiling with -std=c99 */
#define _GNU_SOURCE
#endif