add easyconfigs for older GCC versions

GCC versions prior to v4.5 have no support yet for supplying GMP/MPFR sources, and thus require proper dependencies instead (MPC is not a dependency yet in v4.4)

I also briefly looked into GCC v4.1 and v4.0, but it seems like the system compiler I was using was too new; warning were being generated on code in libcpp, that were being treated as error because of `-Werror`. It seems like an old GCC version (4.2?) should be used as a build dependency in order to get GCC v4.1 to build...

Depends on https://github.com/hpcugent/easybuild-easyblocks/pull/400