build: let HOST_CXXFLAGS default to HOST_CFLAGS

Usage of g++ should mimic the usage of gcc,
otherwise, part of a binary or library may have
optimizations and part of it may not,
unbeknownst to the users or developers working on a build.
This can lead to some features like, for example, FORTIFY_SOURCE,
to have less of an effect or even cause a build error on some hosts.

Therefore, let HOST_CXXFLAGS default to HOST_CFLAGS.

Fixes: 87d489f67 ("build: add HOST_CXXFLAGS for host build")
Signed-off-by: Michael Pratt <[email protected]>
Link: https://github.com/openwrt/openwrt/pull/15853
Signed-off-by: Robert Marko <[email protected]>

rules.mk

@@ -245,8 +245,8 @@ export PKG_CONFIG
 HOSTCC:=$(STAGING_DIR_HOST)/bin/gcc
 HOSTCXX:=$(STAGING_DIR_HOST)/bin/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 $(HOST_CPPFLAGS)
+HOST_CXXFLAGS:=$(HOST_CFLAGS)
 HOST_LDFLAGS:=-L$(STAGING_DIR_HOST)/lib $(if $(IS_PACKAGE_BUILD),-L$(STAGING_DIR_HOSTPKG)/lib -L$(STAGING_DIR)/host/lib)
 
 BUILD_KEY=$(TOPDIR)/key-build