I'm not so sure about the sketch_flags stuff (and I expect it might raise more discussion, as it ties in with previous discussion about setting sketch-specific options that has of yet been unresolved).

However, adding aPLATFORM version define sounds like a sane thing to do. Especially since the IDE and core versions are no longer tied together since quite some time, checkingARDUINO is no longer effective to check for the presence of certain features of bugfixes in the core.

AddingARDUINO_BOARD andARDUINO_ARCH is probably a good idea, though I wonder if that stuff should really be done in arduino-builder with some "default flags" thing (in which case theARDUINO define and some others should be moved as well), to get some consistency in different cores...