diff --git a/callouts/nm-avahi-autoipd-action.c b/callouts/nm-avahi-autoipd-action.c
index 4eec8640d4..440efadee4 100644
--- a/callouts/nm-avahi-autoipd-action.c
+++ b/callouts/nm-avahi-autoipd-action.c
@@ -18,8 +18,6 @@
  * Copyright (C) 2008 Red Hat, Inc.
  */
 
-#define _GNU_SOURCE
-
 #include <stdio.h>
 #include <unistd.h>
 #include <stdlib.h>
diff --git a/callouts/nm-dhcp-client-action.c b/callouts/nm-dhcp-client-action.c
index a1956af545..f001c9e43a 100644
--- a/callouts/nm-dhcp-client-action.c
+++ b/callouts/nm-dhcp-client-action.c
@@ -18,8 +18,6 @@
  * Copyright (C) 2007 - 2008 Red Hat, Inc.
  */
 
-#define _GNU_SOURCE
-
 #include <stdio.h>
 #include <unistd.h>
 #include <stdlib.h>
diff --git a/configure.in b/configure.in
index 9b904b0cf5..97ab091ccd 100644
--- a/configure.in
+++ b/configure.in
@@ -434,6 +434,9 @@ else
 	AC_MSG_RESULT(no)
 fi
 
+# Define _GNU_SOURCE for various things like strcasestr()
+CFLAGS="$CFLAGS -D_GNU_SOURCE"
+
 GTK_DOC_CHECK(1.0)
 
 dnl
diff --git a/libnm-util/tests/Makefile.am b/libnm-util/tests/Makefile.am
index 4ce9511cd5..1cf19cbc46 100644
--- a/libnm-util/tests/Makefile.am
+++ b/libnm-util/tests/Makefile.am
@@ -22,8 +22,7 @@ test_crypto_SOURCES = \
 	test-crypto.c
 
 test_crypto_CPPFLAGS = \
-	$(GLIB_CFLAGS) \
-	-D_GNU_SOURCE
+	$(GLIB_CFLAGS)
 
 test_crypto_LDADD = \
 	$(GLIB_LIBS) \