Silence an unused variable warning from Clang

[ararslan]

Part of my never-ending quest to silence Clang warnings emitted when building Julia.

[yuyichao]

All macros should start with JL_. _ prefix is actually reserved names.

I think we are just using (void) directly. It's a pretty standard C trick and is actually shorter to type than JL_UNUSED().....

[ararslan]

Okay, so rather than introducing a macro for this, I should change _UNUSED(truncret) to (void)truncret?

[yuyichao]

Yes, I think that's good enough. C macros are mainly useful when there's compiler dependent definitions or if the definition is actually complicated. unused is neither. It also conflicts with a potential useful macro that uses the unused attribute to mark unused functions so I was especially avoiding introducing it as a void cast.

[ararslan]

Okay, I'll make that change. Thanks!