IDE 1.6.10 on LinuxMint 17.3 Cinnamon When compiling I get the error 
'as: unknown option »-mmcu=avr5« exit status 1' 

I succeeded now using your piece of advise. Thank you so much.
Delete or rename the old "Arduino15" directory located at /home/(username)/.arduino15    (a.k.a. ~/.arduino15) and re-install 1.6.10.