build: add HOST_CXXFLAGS for host build

Added HOST_CXXFLAGS to specify CXXFLAGS during host-compile
(e.g. to specify c++ standard: HOST_CXXFLAGS += -std=c++11)

Signed-off-by: Sergey V. Lobanov <sergey@lobanov.in>

include/host-build.mk

@@ -51,6 +51,7 @@ HOST_CONFIGURE_VARS = \
 	CFLAGS="$(HOST_CFLAGS)" \
 	CXX="$(HOSTCXX)" \
 	CPPFLAGS="$(HOST_CPPFLAGS)" \
+	CXXFLAGS="$(HOST_CXXFLAGS)" \
 	LDFLAGS="$(HOST_LDFLAGS)" \
 	CONFIG_SHELL="$(SHELL)"