diff --git a/Makefile b/Makefile
index e0bc307..9ed63fc 100644
--- a/Makefile
+++ b/Makefile
@@ -1,7 +1,8 @@
 CC ?= gcc
 DEBUG ?= 0
 
-CFLAGS = -Wall -Werror -Wextra -Wpedantic -std=c99 -Ofast
+CFLAGS = -Wall -Werror -Wextra -Wpedantic -std=c99 -Ofast \
+-D_POSIX_C_SOURCE=200809L
 
 ifeq ($(DEBUG), 1)
 	CFLAGS += -g -DDEBUG