added  -D__PROG_TYPES_COMPAT__ to the avr-g++ compiler flags

this results in just a deprecated warning rather than an error when using libraries that still use prog_XXX types

this is how the IDE does it, and means that libraries don't have to add the ```#define __PROG_TYPES_COMPAT__``` line.

References:
http://www.nongnu.org/avr-libc/user-manual/group__avr__pgmspace.html
https://github.com/jcw/jeelib/commit/f21ac03