Hey ! Hello !
I have not tried to build gcc against 2.4 linux-kernel-headers, you could try to do so following steps on slind toolchain webpage. If you do so, it would be nice to report if it worked for you.
You can also try "buildcross" way to compile it, but in any case you''ll need patching binutils and gcc.
I don't know if using 2.4 linux-kernel-headers would make any difference. I did (with buildcross) for i386|unstable some building, it is at emdebian apt repository. That might help you. (I'll try to build testing too as it is the one you are using). But those are against linux-kernel-headers from a