--- /dev/null
+diff --git a/configure.ac b/configure.ac
+index 466f7f9..0dec17f 100644
+--- a/configure.ac
++++ b/configure.ac
+@@ -27,7 +27,7 @@ AM_INIT_AUTOMAKE([foreign dist-bzip2 no-dist-gzip])
+ AC_CONFIG_HEADERS(config.h)
+ AM_MAINTAINER_MODE
+
+-SET_FEATURES="-D_ISOC99_SOURCE -D_POSIX_C_SOURCE=200112L"
++SET_FEATURES="-D_ISOC99_SOURCE -D_POSIX_C_SOURCE=200112L -D_XOPEN_SOURCE=500"
+
+ case "${host_os}" in
+ "")