This typedef is now
deprecated because the usage of the __progmem__ attribute on a type is not supported in
GCC. However, the use of the __progmem__ attribute on a variable declaration is
supported, and this is now the recommended usage.
The typedef is only visible if the macro __PROG_TYPES_COMPAT__ has been defined
before including <avr/pgmspace.h> (either by a define directive, or by a -D compiler
option.)