Use cfg_if to simplify some cfgs

[dlrobertson]

The cfg_if crate is no_std and use of the cfg_if! macro could simplify some of the code where we have a pattern like the following:

#[cfg(something)]
// Do something
#[cfg(not(something))]
// Do something else

[whitequark]

I vaguely recall some issue with cfg-if as a reason to not use it. But I can't find it now. So I think it's OK.

[dlrobertson]

Looked at this briefly last night and I ran into rust-lang/cfg-if#8. This greatly limits the usefulness of cfg_if for us, but there still may be locations in the codebase that would benefit from the use of cfg_if.

[whitequark]

Oh yeah pretty sure that was the issue. Maybe cfg-if can be fixed?

[dlrobertson]

Yeah it would require some reworking, but I don't see why you couldn't also allow a block or expr token. This is of particular interest to me because the current state of cfgs in src/libcore/ffi.rs for VaList are far from readable, so I'll likely look into this a bit over the next week or so.

[Dirbaio]

I'm going to close this: it's rare that cfg's are paired in cfg(something), cfg(not(something)), other cases aren't made that better by cfg-if, and IMO it's nice if all cfgs are handled the same.