Fix for pgm_read_ptr macro

[nkrkv]

There’s an error in avr/pgmspace.h that leads to errors if one uses pgm_read_ptr macro on SAM platform:

.../cores/arduino/avr/pgmspace.h:106:49: error: 'const void*' is not a pointer-to-object type
 #define pgm_read_ptr(addr) (*(const void *)(addr))
                                                 ^

Given the double indirection (the first is addr and the second is data itself) we should use void**, not void*.

Fixes #35