diff --git a/Tools/GNUMake/Make.defs b/Tools/GNUMake/Make.defs
index 3ab7484b78b..b53fee644e5 100644
--- a/Tools/GNUMake/Make.defs
+++ b/Tools/GNUMake/Make.defs
@@ -452,6 +452,7 @@ CXXFLAGS += $(XTRA_CXXFLAGS)
 CFLAGS   += $(XTRA_CFLAGS)
 FFLAGS   += $(XTRA_FFLAGS)
 F90FLAGS += $(XTRA_F90FLAGS)
+CPPFLAGS += $(XTRA_CPPFLAGS)
 
 ifeq ($(MAKECMDGOALS),check_fortran)
   ifneq ($(lowercase_comp),gnu)