Add option to auto download GMP, MPFR and MPC

Let me know if you want me to change something. I prefer this option to be ON by default.

GCC depends on GMP, MPFR and MPC. However on some systems, notably RHEL/CentOS, those packages are not available in official repositaries or only obsolete versions are available. With this patch arc-versions.sh will download source tarballs of these dependencies and place them inside gcc directory. GCC makefiles are smart enough to use those instead of packages installed on system. Those dependencies will be linked statically inside GCC.