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

This results in just a deprecated warning rather than an error
when using libraries (e.g. jeelib) 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

Reference: http://www.nongnu.org/avr-libc/user-manual/group__avr__pgmspace.html

Fix #169