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)"
 

rules.mk

@@ -240,6 +240,7 @@ export PKG_CONFIG
 HOSTCC:=gcc
 HOSTCXX:=g++
 HOST_CPPFLAGS:=-I$(STAGING_DIR_HOST)/include $(if $(IS_PACKAGE_BUILD),-I$(STAGING_DIR_HOSTPKG)/include -I$(STAGING_DIR)/host/include)
+HOST_CXXFLAGS:=
 HOST_CFLAGS:=-O2 $(HOST_CPPFLAGS)
 HOST_LDFLAGS:=-L$(STAGING_DIR_HOST)/lib $(if $(IS_PACKAGE_BUILD),-L$(STAGING_DIR_HOSTPKG)/lib -L$(STAGING_DIR)/host/lib)