fix __attribute__ for apple broken cpp

[Stan Shebs]

Aldy Hernandez wrote:
> 
> another patch from the stubbort department.
> 
> darwin's cpp expands ATTRIBUTE_UNUSED* macros incorrectly.  here is a patch
> to fix it.  i'm really hoping later releases of the apple development tools
> will have the preprocessor fixed.

The weird preprocessor is going to be retired.

> the current __APPLE_CC__ magic number is 934 for my 10.1 distribution.  i'm
> hoping they don't take too long, else i'll have to up the magic number below.

We can't make *any* promises like that, because those numbers are
tied to our group's internal submissions to OS X.  Three bad bugs
in a row will result in three submissions and an increment of three
in the number, for instance.  There's no way that this patch can
be reliable.

Stan