Add option to auto download GMP, MPFR and MPC

GCC depends on GMP, MPFR and MPC. However on some systems, notably
RHEL/CentOS, those packages are not available in official repositories 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.

I've checked this script on Ubuntu 13.04 and RHEL 6.4.