diff --git a/options.mk b/options.mk
index eee2334..311024c 100644
--- a/options.mk
+++ b/options.mk
@@ -14,6 +14,6 @@ INCS = -I${X11INC} -I${FREETYPEINC} -I${YAJLINC} `pkg-config --cflags xft pango
 LIBS = -L${X11LIB} -lX11 ${XINERAMALIBS} ${FREETYPELIBS} -lXrender ${IMLIB2LIBS} -lX11-xcb -lxcb -lxcb-res -lXext ${YAJLLIBS} `pkg-config --libs xft pango pangoxft`
 
 # flags
-CPPFLAGS = -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_POSIX_C_SOURCE=200809L -DVERSION=\"${VERSION}\" ${XINERAMAFLAGS}
+CPPFLAGS = -D_DEFAULT_SOURCE -D_BSD_SOURCE -D_XOPEN_SOURCE=700L -DVERSION=\"${VERSION}\" ${XINERAMAFLAGS}
 
 # NOTE: host.mk and toggle.mk will both override this.