build: pass HOSTCXX to host builds as CXX

Fixes cmake build on some systems that also have an older clang++

Signed-off-by: Felix Fietkau <nbd@nbd.name>

include/host-build.mk

@@ -52,6 +52,7 @@ endef
 HOST_CONFIGURE_VARS = \
 	CC="$(HOSTCC)" \
 	CFLAGS="$(HOST_CFLAGS)" \
+	CXX="$(HOSTCXX)" \
 	CPPFLAGS="$(HOST_CPPFLAGS)" \
 	LDFLAGS="$(HOST_LDFLAGS)" \
 	CONFIG_SHELL="$(SHELL)"