Allow casting a flag to a type in libc_bitflags!
Allow casting a flag to a type in libc_bitflags!

This is necessary because certain flags in libc have different types, generally
due to a mistake when adding the flags to the libc. See
https://github.com/rust-lang/libc/pull/511 for an example of such a
discrepency.

MarkusJais/nix-rust,geofft/nix-rust,xd009642/nix,berkowski/nix,nix-rust/nix,geofft/nix-rust,xd009642/nix,carllerche/nix-rust,asomers/nix,carllerche/nix-rust,berkowski/nix,MarkusJais/nix-rust,Susurrus/nix,nix-rust/nix,Susurrus/nix,MarkusJais/nix-rust,xd009642/nix,berkowski/nix,geofft/nix-rust,asomers/nix,Susurrus/nix,carllerche/nix-rust