diff --git a/config.mk b/config.mk
index 6a37cc8..0064496 100644
--- a/config.mk
+++ b/config.mk
@@ -11,7 +11,7 @@ INCS =
 LIBS =
 
 # flags
-CPPFLAGS = -DVERSION=\"${VERSION}\"
+CPPFLAGS = -DVERSION=\"${VERSION}\" -D_XOPEN_SOURCE=500
 CFLAGS = -std=c99 -pedantic -Wall -Os ${INCS} ${CPPFLAGS}
 LDFLAGS = -s ${LIBS}
 
diff --git a/slow.c b/slow.c
index 71b1fef..da46795 100644
--- a/slow.c
+++ b/slow.c
@@ -1,4 +1,3 @@
-#define _XOPEN_SOURCE 500
 #include <stdlib.h>
 #include <stdio.h>
 #include <unistd.h>