diff --git a/Makefile b/Makefile
index 542d180..6fa9cf8 100644
--- a/Makefile
+++ b/Makefile
@@ -1,4 +1,4 @@
-CFLAGS?=-Wall -Os
+CFLAGS?=-Wall -Os -std=c99
 LDADD?=`pkg-config --cflags --libs x11 xinerama`
 
 normal:
@@ -12,4 +12,4 @@ docs:
 clean:
 	rm -f xoat xoat-debug
 
-all: docs normal
\ No newline at end of file
+all: docs normal