Restore BUFFER_T_DEFINED legacy check flag in HalideRuntime.h
Restore BUFFER_T_DEFINED legacy check flag in HalideRuntime.h

Restoring this flag would be an enormous help to those of us who are updating our embarrassingly Australopithecine codebases to work with the new `halide_buffer_t` and other modern Halide runtime things, and I don’t believe it would break anything, no would it exacerbate anyones’ reliance on legacy `buffer_t` use.

Former-commit-id: 1463a3da591623e913591dc83aaa789ff46e5925
darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide,darkbuck/Halide,darkbuck/Halide,Trass3r/Halide,Trass3r/Halide,darkbuck/Halide