Don't always define __PROG_TYPES_COMPAT__ in CPPFLAGS

[MachFour]

First of all, thank you for making such a useful build tool!

I noticed that the CPPFLAGS in Arduino.mk (line 1016) unconditionally defines __PROG_TYPES_COMPAT__, which seems unnecessary since it is deprecated and not needed to compile the latest version of the Arduino core libs.

Could Arduino.mk be changed to not always define __PROG_TYPES_COMPAT__? Even a Make variable to disable that define would be fine, though IMO I think it should be disabled by default.