Fix Arduino Mega 2560 avr-libc issue

Arduino MEGA2560_09   If you have Arduino Mega 2560 on any updated Linux distro with avr-libc 1.8 you probably have an issue with compiling.  

Compiler usually fails with this error: attempt to use poisoned "SIGUSART0RECV"
  Quick fix is to downgrade to earlier version of avr-libc...
  Grab older version of avr-libc from Koji:
wget http://kojipkgs.fedoraproject.org//packages/avr-libc/1.7.1/4.fc18/noarch/avr-libc-1.7.1-4.fc18.noarch.rpm
  Now delete problematic avr-libc 1.8:
sudo rpm -e avr-libc --nodeps
  And finally install avr-libc 1.7:
sudo yum localinstall avr-libc-1.7.1-3.fc17.noarch.rpm
  Done.