pert:
Are you sure? Arduino AVR Boards 1.6.12 does install avrdude 6.3.0-arduino2 and cause it to be used by the Arduino IDE
Oh, I didn't realise that installing "Arduino AVR Boards" V1.6.12 also installed avrdude 6.3.0-arduino2. And since IDE V1.6.9 has avrdude V6.01 by default, I just assumed that it was being used. My bad. (When I typed the reply earlier, I'd already rolled back to "Arduino AVR Boards" 1.6.11, so couldn't double-check.)
We did make a fix to the ATTinyCore Boards Manager file so it shouldn't cause any problems with Arduino IDE 1.6.10 now. As to whether the new versions of avr-gcc and avrdude will cause issues with that core, I'm not sure.
I wasn't aware of this. I had a look at the 'spencekonde' ATtiny cores Github webpage yesterday, and there was still a warning to use IDE V1.6.9 and not IDE V1.6.10.
Anyway, since IDE V1.6.10 still seems to have bugs, particularly the one relating to not being able to upload if the *.ino file is directly clicked on to open the IDE, I'll stick with IDE V1.6.9 for now.
It would be lovely to see some of these things sorted out over the next one or two releases. Fingers crossed. 