This commit expands the conditional compilation directives to support
building Alacritty for OpenBSD. The build succeeds, and Alacritty runs
without issue once https://github.com/rust-lang/libc/pull/957 has been
merged and added to a versioned libc release.
This has been tested on the recently-released OpenBSD 6.3 on amd64 with
rustc 1.24.0 from its standard ports tree.
Because there are so many clippy warnings in the current codebase,
this commit removes '#![cfg_attr(feature = "clippy", deny(clippy))]',
to make it easier to fix warnings incrementally.
