diff --git a/3rdparty/everest/library/Hacl_Curve25519_joined.c b/3rdparty/everest/library/Hacl_Curve25519_joined.c
index 149e4741a..957294f64 100644
--- a/3rdparty/everest/library/Hacl_Curve25519_joined.c
+++ b/3rdparty/everest/library/Hacl_Curve25519_joined.c
@@ -19,9 +19,12 @@
  *  This file is part of mbed TLS (https://tls.mbed.org)
  */
 #ifndef _BSD_SOURCE
-#define _BSD_SOURCE /* Required to enable compilation under --std=c99 */
+/* Required to get htole64() from gcc/glibc's endian.h (older systems)
+ * when we compile with -std=c99 */
+#define _BSD_SOURCE
 #endif
 #ifndef _DEFAULT_SOURCE
+/* (modern version of _BSD_SOURCE) */
 #define _DEFAULT_SOURCE
 #endif