CFLAGS for the host in canadian cross build

[Max Filippov]

Hello,

I'm trying to build a canadian cross compiler with build == x86_64-linux,
host == xtensa-linux and target == xtensa-linux. I need to specify an
xtensa-specific flag in CFLAGS that will be applied to the host binary.

I put this flag into CFLAGS and CXXFLAGS. The first question: is that
the right place for it?

The build process tries to build libcpp for the build system and it runs
x86_64 gcc with that xtensa-specific option. That's where the build
breaks for me, because the option is not recognized by the build gcc.
Is it supposed to work?

If CFLAGS/CXXFLAGS is the right place for the option, how can the
build tell whether it's intended for the build or for the host?
-- 
Thanks.
-- Max