Add include guards to avr/pgmspace.h
Add empty implementation of analogReference()
Add dummy macros in avr/interrupt.h

This should allow a sketch that uses low-level interrupts to compile,
even though the interrupts are never called.
Add dummy WInterrupts.c

This allows sketches that use attachInterrupt() and detachInterrupt() to
compile, even though the interrupts themselves will never be called.
Add Tone.cpp with empty functions
Make sure Makefile.mk doesn't set the default goal

Before, you would need to add the default goal before including
Arduino.mk, since the default goal would otherwise become the first rule
in Arduino.mk. Now, Arduino.mk explicitely resets the default goal so
the default goal is again the first goal in the main Makefile.
Add README and LGPL license text