How to specify CFLAGS, CXXFLAGS, CPPFLAGS and LDFLAGS in the stage 2 or later when building libraries?

[bootmgr--- via Gcc-help <gcc-help@xxxxxxxxxxx>]

Hi,
How can I add any flags for libbacktrace, libcody, libcpp, etc in stage 2 or later?
I try to use CFLAGS_FOR_TARGET and CFLAGS_FOR_BUILD, but it didn't work.