diff --git a/Makefile b/Makefile
index 42e8dac..2678580 100644
--- a/Makefile
+++ b/Makefile
@@ -31,12 +31,12 @@ endif
 FREESTANDING := no
 
 ifeq ($(FREESTANDING),yes)
-	CFLAGS += -DFREESTANDING
+	CPPFLAGS += -DFREESTANDING
 	EXPORT_UNPREFIXED = no
 endif
 
 ifeq ($(EXPORT_UNPREFIXED),yes)
-	CFLAGS += -DEXPORT_UNPREFIXED
+	CPPFLAGS += -DEXPORT_UNPREFIXED
 endif
 
 LIBUCONTEXT_C_SRC = $(wildcard arch/${ARCH}/*.c)