diff --git a/hermit/tools/Makefile b/hermit/tools/Makefile
index 8fb5bc2ef..2551a9617 100644
--- a/hermit/tools/Makefile
+++ b/hermit/tools/Makefile
@@ -1,6 +1,6 @@
 MAKE = make
 CC = gcc
-CFLAGS = -O2 -Wall
+CFLAGS = -O2 -Wall -std=gnu99
 PROXYFILES = proxy init.sh $(shell find ../usr/tests ../usr/benchmarks ../usr/openmpbench -type f -executable)
 
 # Prettify output