Per-compiler(-class?) attribute pass-thru#61

A scan of the issues list shows a bunch of requests for very specific per-compiler attributes. Worse yet, I just came up against the problem myself, with a GCC attribute I would like to pass through, but of course only in GCC and GCC-emulating hosts. Hedley, help me! I'd like a pass-through macro that works for a target platform. Something like: HEDLEY_IF(GCC, alloc_size(3)) The point would be to pass through the attribute only on GCC hosts. It should be fairly easy to extend to MSVC, IAR, etc. by changing the first argument. Basically, if you have an attribute that only works on target X, it should be possible to have it active only on target X with Hedley, without having to do the `#ifdef` dance.