<avr/version.h>: avr-libc version macros

This header file defines macros that contain version numbers and strings describing the current version of avr-libc.

The version number itself basically consists of three pieces that are separated by a dot: the major number, the minor number, and the revision number. For development versions (which use an odd minor number), the string representation additionally gets the date code (YYYYMMDD) appended.

This file will also be included by <avr/io.h>. That way, portable tests can be implemented using <avr/io.h> that can be used in code that wants to remain backwards-compatible to library versions prior to the date when the library version API had been added, as referenced but undefined C preprocessor macros automatically evaluate to 0.