Cross compiling GNU toolchain.
 

I am currently learning about how to build a cross compiling GNU toolchain. I have managed to get a toolchain built using the instructions in Bill Gatliff's excellent Dreamcast article. However I am confused about the bootstrap compiler step. Why does a compiler that is running on the host need to have access to files relating to the C library for the target hardware before it can be fully built? If anyone can enlighten me I would be most grateful.