diff --git a/Makefile.include b/Makefile.include
index 782d2d5..5e48ba0 100644
--- a/Makefile.include
+++ b/Makefile.include
@@ -14,6 +14,7 @@ DBGFLAGS ?= -g -DNDEBUG
 
 CFLAGS   += $(OPTFLAGS) \
             $(DBGFLAGS) \
+            -std=c99 \
             -W \
             -Wall \
             -Wextra \