diff --git a/CMakeLists.txt b/CMakeLists.txt
index 98082cde..5a49875c 100644
--- a/CMakeLists.txt
+++ b/CMakeLists.txt
@@ -334,7 +334,7 @@ if (NOT DEFINED _GNU_SOURCE)
 endif()
 
 if (_GNU_SOURCE)
-    add_definitions(-D_GNU_SOURCE)
+    add_definitions(-D_GNU_SOURCE=1)
 endif()
 
 CHECK_INCLUDE_FILE(sys/types.h EVENT__HAVE_SYS_TYPES_H)
diff --git a/configure.ac b/configure.ac
index 0f5495d6..b8a7839d 100644
--- a/configure.ac
+++ b/configure.ac
@@ -478,7 +478,7 @@ fi
 AC_MSG_CHECKING(for F_SETFD in fcntl.h)
 AC_EGREP_CPP(yes,
 [
-#define _GNU_SOURCE
+#define _GNU_SOURCE 1
 #include <fcntl.h>
 #ifdef F_SETFD
 yes
@@ -667,7 +667,7 @@ AC_CHECK_SIZEOF(off_t)
 AC_CHECK_SIZEOF(time_t)
 
 AC_CHECK_TYPES([struct in6_addr, struct sockaddr_in6, sa_family_t, struct addrinfo, struct sockaddr_storage], , ,
-[#define _GNU_SOURCE
+[#define _GNU_SOURCE 1
 #include <sys/types.h>
 #ifdef HAVE_NETINET_IN_H
 #include <netinet/in.h>
diff --git a/include/event2/util.h b/include/event2/util.h
index 7c872737..e6df6289 100644
--- a/include/event2/util.h
+++ b/include/event2/util.h
@@ -58,9 +58,6 @@ extern "C" {
 #endif
 #include <stdarg.h>
 #ifdef EVENT__HAVE_NETDB_H
-#if !defined(_GNU_SOURCE)
-#define _GNU_SOURCE
-#endif
 #include <netdb.h>
 #endif