diff --git a/cppflags-iterator.mk b/cppflags-iterator.mk
index 5f3bcc0668..790d345a0a 100644
--- a/cppflags-iterator.mk
+++ b/cppflags-iterator.mk
@@ -4,4 +4,4 @@
 cpp-src := $(firstword $(cpp-srcs-left))
 cpp-srcs-left := $(filter-out $(cpp-src),$(cpp-srcs-left))
 
-CPPFLAGS-$(cpp-src) = -DNOT_IN_libc -DIS_IN_$(lib)
+CPPFLAGS-$(cpp-src) += -DNOT_IN_libc -DIS_IN_$(lib)