[Bug libstdc++/53579] libstdc++ configure use CXXFLAGS instead of CXXFLAGS_FOR_TARGET

[anatol.pomozov at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=53579

--- Comment #3 from Anatol <anatol.pomozov at gmail dot com> ---
I just hit this issue when tried to build cross-tools for arm64.
CFLAGS_FOR_TARGET works as expected and I was assuming that CXXFLAGS_FOR_TARGET
is used instead of CXXFLAGS.

If CXXFLAGS_FOR_TARGET is not honored now, what is recommended workaround for
cross-compilers that need different CXXFLAGS for host and target?