_POSIX_C_SOURCE=200809L

config.mk

@@ -17,7 +17,7 @@ INCS = -I${X11INC}
 LIBS = -L${X11LIB} -lX11 ${XINERAMALIBS}
 
 # flags
-CPPFLAGS = -D_BSD_SOURCE -DVERSION=\"${VERSION}\" ${XINERAMAFLAGS}
+CPPFLAGS = -D_BSD_SOURCE -D_POSIX_C_SOURCE=200809L -DVERSION=\"${VERSION}\" ${XINERAMAFLAGS}
 CFLAGS   = -ansi -pedantic -Wall -Os ${INCS} ${CPPFLAGS}
 LDFLAGS  = -s ${LIBS}