diff --git a/lib/tls/openssl/openssl-client.c b/lib/tls/openssl/openssl-client.c
index dc69985a2..926e70a74 100644
--- a/lib/tls/openssl/openssl-client.c
+++ b/lib/tls/openssl/openssl-client.c
@@ -25,8 +25,10 @@
 #include "lws_config.h"
 #ifdef LWS_HAVE_X509_VERIFY_PARAM_set1_host
 /* Before glibc 2.10, strnlen required _GNU_SOURCE */
+#if !defined(_GNU_SOURCE)
 #define _GNU_SOURCE
 #endif
+#endif
 #include <string.h>
 
 #include "private-lib-core.h"