For starters, there are all the circular dependencies:
• GCC includes libgcc and so depends on glibc
• glibc of course cannot be built without GCC, and it also depends on MIG, as well as Mach and Hurd headers
• MIG depends on Mach too, for types & stuff
• Mach naturally needs GCC and MIG to be built
• The Hurd needs all four — GCC, MIG, Mach, and glibc.
Yay