devel/rust-cbindgen: Fix rust errno binding.

It looks like that rust-random/getrandom#54 has
wrongly assigned us to Darwin case.  The __error() is not libc callable
function, but rather inline helper implemented at headers level to assist
code that was using errno incorrectly.
It seems that thread_local has still not become stable feature in rust?
If this very useful language feature does not get official support soon
enough in rust, it would be trivial for us to add simple helper function
in libc using the most common __errno_location() variant out there.

Since I could not figure out how to disable -Werror on this warning when
using "extern { #[thread_local] static errno: c_int; }", just bringing
back the previous crate helper that worked (using google search).

ports/devel/rust-cbindgen/Makefile.DragonFly

@@ -1,9 +1,2 @@
-#
-# XXX - The current logic in Mk/Uses/cargo.mk will try to append
-# LDFLAGS to RUSTFLAGS (via -C link-arg) but it turns out that as
-# we don't define LDFLAGS for this port, it will pass an empty arg
-# which rustc converts into "" and gcc doesn't like that. It produces
-# a 'No such file or directory' error.
-#
-# Add a dummy argument to the $CC call so that the build succeeds.
-LDFLAGS="-v"
+
+CARGO_CRATES+=	gcc-0.3.55 errno-dragonfly-0.1.1

ports/devel/rust-cbindgen/diffs/distinfo.diff

@@ -0,0 +1,10 @@
+--- distinfo.orig	2019-08-28 19:53:49 UTC
++++ distinfo
+@@ -73,3 +73,7 @@ SHA256 (rust/crates/winapi-i686-pc-windo
+ SIZE (rust/crates/winapi-i686-pc-windows-gnu-0.4.0.tar.gz) = 2918815
+ SHA256 (rust/crates/winapi-x86_64-pc-windows-gnu-0.4.0.tar.gz) = 712e227841d057c1ee1cd2fb22fa7e5a5461ae8e48fa2ca79ec42cfc1931183f
+ SIZE (rust/crates/winapi-x86_64-pc-windows-gnu-0.4.0.tar.gz) = 2947998
++SHA256 (rust/crates/gcc-0.3.55.tar.gz) = 8f5f3913fa0bfe7ee1fd8248b6b9f42a5af4b9d65ec2dd2c3c26132b950ecfc2
++SIZE (rust/crates/gcc-0.3.55.tar.gz) = 37262
++SHA256 (rust/crates/errno-dragonfly-0.1.1.tar.gz) = 14ca354e36190500e1e1fb267c647932382b54053c50b14970856c0b00a35067
++SIZE (rust/crates/errno-dragonfly-0.1.1.tar.gz) = 1370

ports/devel/rust-cbindgen/dragonfly/patch-temp_errno_fixup

@@ -0,0 +1,37 @@
+--- cargo-crates/getrandom-0.1.11/Cargo.toml.intermediate	2019-09-06 14:30:19.000000000 +0000
++++ cargo-crates/getrandom-0.1.11/Cargo.toml
+@@ -33,6 +33,9 @@ version = "1.0"
+ optional = true
+ package = "rustc-std-workspace-core"
+
++[dependencies.errno-dragonfly]
++version = "0.1.1"
++
+ [dependencies.log]
+ version = "0.4"
+ optional = true
+--- cargo-crates/getrandom-0.1.11/src/util_libc.rs.orig	2019-08-24 23:12:51 UTC
++++ cargo-crates/getrandom-0.1.11/src/util_libc.rs
+@@ -10,6 +10,7 @@ use crate::util::LazyUsize;
+ use crate::Error;
+ use core::num::NonZeroU32;
+ use core::ptr::NonNull;
++extern crate errno_dragonfly;
+
+ cfg_if! {
+     if #[cfg(any(target_os = "netbsd", target_os = "openbsd", target_os = "android"))] {
+@@ -18,10 +19,13 @@ cfg_if! {
+         use libc::__errno_location as errno_location;
+     } else if #[cfg(any(target_os = "solaris", target_os = "illumos"))] {
+         use libc::___errno as errno_location;
+-    } else if #[cfg(any(target_os = "macos", target_os = "freebsd", target_os = "dragonfly"))] {
++    } else if #[cfg(any(target_os = "macos", target_os = "freebsd"))] {
+         use libc::__error as errno_location;
+     } else if #[cfg(target_os = "haiku")] {
+         use libc::_errnop as errno_location;
++    } else if #[cfg(target_os = "dragonfly")] {
++        // soon will be use libc::__errno_location as errno_location;
++        use errno_dragonfly::errno_location;
+     }
+ }
+