As of now, gcc builds with gmp, mpfr, and mpc source directories placed in the gcc tree by the user.  Adding the other two main prerequisites into the tree for full gcc features would be a win for users.  The inter-dependence of the configuration options between the latter two and gmp, mpfr, and mpc makes it difficult for a user to build all successfully without trial and error.  An explicitly versioned set of the five sources known to work for a given version of gcc to be downloaded with a helper script (like the one by Andrew Haley: download_prerequisites.sh) would be very helpful and ease debugging and help for all.