Fix support for DragonFly

* DragonFly does not have a O_DSYNC flag
* Fix type_of_cmsg_data on DragonFly
* No fexecve() on DragonFly
* Do not run aio test cases on DragonFly
* Keep target lists in alphabetical order
* Unscrable #[cfg] directives and use cfg_if! macro instead
* Fix errno on DragonFly

Below follows an explanation why we have to use a C extension
to get errno working on DragonFly:

DragonFly uses a thread-local errno variable, but #[thread_local] is
feature-gated and not available in stable Rust as of this writing (Rust
1.21.0). We have to use a C extension (src/errno_dragonfly.c) to access
it.

Tracking issue for `thread_local` stabilization:

    https://github.com/rust-lang/rust/issues/29594

Once this becomes stable, we can remove build.rs, src/errno_dragonfly.c,
remove the build-dependency from Cargo.toml, and use:

    extern {
        #[thread_local]
        static errno: c_int;
    }

Now all targets will use the build.rs script, but only on DragonFly this
will do something. Also, there are no additional dependencies for
targets other than DragonFly (no gcc dep).

1 files changed, 3 insertions, 0 deletions

diff --git a/Cargo.toml b/Cargo.toml
index f9e37545..6eff225e 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -18,6 +18,9 @@ bitflags = "1.0"
 cfg-if = "0.1.0"
 void = "1.0.2"
 
+[target.'cfg(target_os = "dragonfly")'.build-dependencies]
+gcc = "0.3"
+
 [dev-dependencies]
 lazy_static = "1"
 rand = "0.3.8"