diff --git a/Makefile.in b/Makefile.in
--- a/Makefile.in
+++ b/Makefile.in
@@ -170,6 +170,7 @@
 # built for the build system to override those in BASE_FLAGS_TO_PASS.
 EXTRA_BUILD_FLAGS = \
 	CFLAGS="$(CFLAGS_FOR_BUILD)" \
+	CXXFLAGS="$(CXXFLAGS_FOR_BUILD)" \
 	LDFLAGS="$(LDFLAGS_FOR_BUILD)"
 
 # This is the list of directories to built for the host system.
@@ -710,6 +711,7 @@
 	"CC_FOR_BUILD=$(CC_FOR_BUILD)" \
 	"CFLAGS_FOR_BUILD=$(CFLAGS_FOR_BUILD)" \
 	"CXX_FOR_BUILD=$(CXX_FOR_BUILD)" \
+	"CXXFLAGS_FOR_BUILD=$(CXXFLAGS_FOR_BUILD)" \
 	"EXPECT=$(EXPECT)" \
 	"FLEX=$(FLEX)" \
 	"INSTALL=$(INSTALL)" \
diff --git a/Makefile.tpl b/Makefile.tpl
index 1ea1954..78a59c3 100644
--- a/Makefile.tpl
+++ b/Makefile.tpl
@@ -172,6 +172,7 @@
 # built for the build system to override those in BASE_FLAGS_TO_PASS.
 EXTRA_BUILD_FLAGS = \
 	CFLAGS="$(CFLAGS_FOR_BUILD)" \
+	CXXFLAGS="$(CXXFLAGS_FOR_BUILD)" \
 	LDFLAGS="$(LDFLAGS_FOR_BUILD)"
 
 # This is the list of directories to built for the host system.