How to edit cross-compilation options?

[Barracuda09]

Moved from #59

I always can't succeed.
Can you give an example?

export INCLUDES=--sysroot=dir ( What does this do? )
export CXXPREFIX=mipsel-unknown-linux-gnu-gcc ( is ok? )
export CXXSUFFIX=mipsel-unknown-linux-gnu-gcc ( is ok? )
make debug

PS: make clean Can't remove export value