diff --git a/tools/Makefile b/tools/Makefile
index 9e40bcd27..57a4d84ce 100644
--- a/tools/Makefile
+++ b/tools/Makefile
@@ -1,5 +1,7 @@
 .PHONY: all
 
+CFLAGS := -std=c99
+
 all: \
 	lzcomp \
 	png_dimensions \
@@ -11,4 +13,4 @@ all: \
 	@:
 
 %: %.c
-	$(CC) -o $@ $<
+	$(CC) $(CFLAGS) -o $@ $<
diff --git a/tools/gfx.c b/tools/gfx.c
index 25de45dbe..0e6864e91 100644
--- a/tools/gfx.c
+++ b/tools/gfx.c
@@ -3,6 +3,7 @@
 #include <stdbool.h>
 #include <getopt.h>
 #include <string.h>
+#include <stdint.h>
 
 #include "common.h"