_GNU_SOURCE needs to be defined before any standard header.

crypto/dso/dso_dlfcn.c

@@ -56,6 +56,16 @@
  *
  */
 
+/* We need to do this early, because stdio.h includes the header files
+   that handle _GNU_SOURCE and other similar macros.  Defining it later
+   is simply too late, because those headers are protected from re-
+   inclusion.  */
+#ifdef __linux
+# ifndef _GNU_SOURCE
+#  define _GNU_SOURCE	/* make sure dladdr is declared */
+# endif
+#endif
+
 #include <stdio.h>
 #include "cryptlib.h"
 #include <openssl/dso.h>
@@ -69,11 +79,6 @@ DSO_METHOD *DSO_METHOD_dlfcn(void)
 
 #ifdef HAVE_DLFCN_H
 
-#ifdef __linux
-# ifndef _GNU_SOURCE
-#  define _GNU_SOURCE	/* make sure dladdr is declared */
-# endif
-#endif
 #include <dlfcn.h>
 #endif