diff --git a/configure.ac b/configure.ac
index 08c73295..ef679128 100644
--- a/configure.ac
+++ b/configure.ac
@@ -67,6 +67,10 @@ AC_CONFIG_FILES(m4_normalize(m4_foreach_w(THIS_PLUGIN,AVAILABLE_NCP_PLUGINS,[src
 PLUGIN_SUBDIRS="[]m4_foreach_w(THIS_PLUGIN,AVAILABLE_NCP_PLUGINS,ncp-[]THIS_PLUGIN )"
 AC_SUBST(PLUGIN_SUBDIRS)
 
+AC_USE_SYSTEM_EXTENSIONS
+AC_GNU_SOURCE
+CFLAGS="$CFLAGS -D_GNU_SOURCE"
+
 AM_INIT_AUTOMAKE()
 
 AM_MAINTAINER_MODE
diff --git a/src/util/nlpt-select.c b/src/util/nlpt-select.c
index b8b1c600..ebd76199 100644
--- a/src/util/nlpt-select.c
+++ b/src/util/nlpt-select.c
@@ -24,8 +24,12 @@
 #include <config.h>
 #endif
 
+#ifndef _XOPEN_SOURCE
 #define _XOPEN_SOURCE 1 // For the "fds_bits" member of "fd_set"
+#ifndef __USE_XOPEN
 #define __USE_XOPEN   1 // This too
+#endif // __USE_XOPEN
+#endif // _XOPEN_SOURCE
 
 #include <stdio.h>
 #include <stdint.h>
diff --git a/src/util/socket-utils.c b/src/util/socket-utils.c
index 07f5b29e..3d7c9c49 100644
--- a/src/util/socket-utils.c
+++ b/src/util/socket-utils.c
@@ -21,8 +21,6 @@
 #include <config.h>
 #endif
 
-#define _GNU_SOURCE 1
-
 #include "assert-macros.h"
 
 #include <stdio.h>
@@ -90,7 +88,7 @@ system_socket_table_close_alarm_(int sig)
 	static const char message[] = "\nclose_super_socket: Unable to terminate child in a timely manner, watchdog fired\n";
 
 	// Can't use syslog here, write to stderr instead.
-	(void)write(STDERR_FILENO, message, sizeof(message) - 1);
+	ssize_t len = write(STDERR_FILENO, message, sizeof(message) - 1);
 
 	_exit(EXIT_FAILURE);
 }
diff --git a/src/util/string-utils.c b/src/util/string-utils.c
index 2dc01a76..d86e085d 100644
--- a/src/util/string-utils.c
+++ b/src/util/string-utils.c
@@ -26,13 +26,15 @@
 #endif
 
 #include <stdio.h>
-#include "string-utils.h"
 #include <ctype.h>
 #include <inttypes.h>
 #include <stdlib.h>
 
+#ifndef __USE_GNU
 #define __USE_GNU // Needed for `strcasestr`
-#include <string.h>
+#endif
+
+#include "string-utils.h"
 
 void
 memcpyrev(void *dest_, const void *src_, size_t len)