This is a big and long overdue improvement, thanks.

I'd be tempted to drop the whole discussion of needed tools and (especially) minimum versions and just point to the wiki, since having a second copy here just means that it's likely to get obsolete again.  It doesn't buy much to have it here either, IMO, since it seems pretty unlikely that someone would be trying to install gem5 for the first time without internet access...

Ship It!