Ok, I have a solution with Coding Badly's help.
See reply #1. Follow all the steps, ignoring the ATtiny xx stuff in the middle.
This seems to be a more direct link to WinAVR
When done moving & renaming the directory, it should compile okay.
I even copied the ATMEGA328P section into a new ATMEGA328 section and changed the last signature byte from 0x0F to 0x14, and compiles now too.
And the Leonardo16 stuff I made for Atmega16U4 compiles now too.