build: properly pass CPP and CXX flags in HOST_MAKE_VARS

Signed-off-by: Jo-Philipp Wich <jo@mein.io>

include/host-build.mk

@@ -69,7 +69,8 @@ HOST_CONFIGURE_ARGS = \
 
 HOST_MAKE_VARS = \
 	CFLAGS="$(HOST_CFLAGS)" \
-	CXXFLAGS="$(HOST_CPPFLAGS)" \
+	CPPFLAGS="$(HOST_CPPFLAGS)" \
+	CXXFLAGS="$(HOST_CXXFLAGS)" \
 	LDFLAGS="$(HOST_LDFLAGS)"
 
 HOST_MAKE_FLAGS =