diff --git a/Makefile.rules b/Makefile.rules
index 241686e..0ab879a 100644
--- a/Makefile.rules
+++ b/Makefile.rules
@@ -30,8 +30,8 @@ endif
 
 AFLAGS	 += $(ARCH)
 CFLAGS	 += $(ARCH) -Wall -Wno-psabi -fsigned-char -fno-builtin -Ofast -DNDEBUG
-CPPFLAGS += $(CFLAGS) -fno-exceptions -fno-rtti -std=c++0x -Wno-write-strings
-CFLAGS	 += -fno-delete-null-pointer-checks -fdata-sections -ffunction-sections -u _printf_float
+CPPFLAGS := $(CFLAGS) $(CPPFLAGS) -fno-exceptions -fno-rtti -std=c++0x -Wno-write-strings
+CFLAGS	 += -fno-delete-null-pointer-checks -fdata-sections -ffunction-sections -u _printf_float -std=gnu99
 
 .PHONY: clean