diff --git a/Makefile b/Makefile
index 43b0c0b..e567b37 100644
--- a/Makefile
+++ b/Makefile
@@ -2,7 +2,7 @@
 
 CC=clang
 LD=clang
-CFLAGS=-Wall -Wextra -Wpedantic -O0 -g3 -std=c23 -fno-omit-frame-pointer -fno-optimize-sibling-calls -D_GNU_SOURCE
+CFLAGS=-Wall -Wextra -Wpedantic -O0 -g3 -std=c23 -fno-omit-frame-pointer -fno-optimize-sibling-calls -D_POSIX_C_SOURCE=200809L
 LDFLAGS?=
 
 SOURCES = $(shell find src/ -type f -name '*.c')