Avoid build failures of unsupported examples on freebsd

I'm not really happy with the readability, there ought to be a real
way to mark an example as "non applicable in cfg so and so".

Signed-off-by: Yann Dirson <yann.dirson@vates.fr>

Author: ydirson

examples/add_netns.rs

@@ -1,8 +1,13 @@
 // SPDX-License-Identifier: MIT
 
+#[cfg(not(target_os = "freebsd"))]
 use rtnetlink::NetworkNamespace;
 use std::env;
 
+#[cfg(target_os = "freebsd")]
+fn main() -> () {}
+
+#[cfg(not(target_os = "freebsd"))]
 #[tokio::main]
 async fn main() -> Result<(), String> {
     env_logger::init();

examples/add_netns_async.rs

@@ -1,8 +1,13 @@
 // SPDX-License-Identifier: MIT
 
+#[cfg(not(target_os = "freebsd"))]
 use rtnetlink::NetworkNamespace;
 use std::env;
 
+#[cfg(target_os = "freebsd")]
+fn main() -> () {}
+
+#[cfg(not(target_os = "freebsd"))]
 #[async_std::main]
 async fn main() -> Result<(), String> {
     env_logger::init();

examples/add_tc_qdisc_ingress.rs

@@ -4,6 +4,10 @@ use std::env;
 
 use rtnetlink::new_connection;
 
+#[cfg(target_os = "freebsd")]
+fn main() -> () {}
+
+#[cfg(not(target_os = "freebsd"))]
 #[tokio::main]
 async fn main() -> Result<(), ()> {
     env_logger::init();

examples/del_netns.rs

@@ -1,8 +1,13 @@
 // SPDX-License-Identifier: MIT
 
+#[cfg(not(target_os = "freebsd"))]
 use rtnetlink::NetworkNamespace;
 use std::env;
 
+#[cfg(target_os = "freebsd")]
+fn main() -> () {}
+
+#[cfg(not(target_os = "freebsd"))]
 #[tokio::main]
 async fn main() -> Result<(), String> {
     let args: Vec<String> = env::args().collect();

examples/del_netns_async.rs

@@ -1,8 +1,13 @@
 // SPDX-License-Identifier: MIT
 
+#[cfg(not(target_os = "freebsd"))]
 use rtnetlink::NetworkNamespace;
 use std::env;
 
+#[cfg(target_os = "freebsd")]
+fn main() -> () {}
+
+#[cfg(not(target_os = "freebsd"))]
 #[async_std::main]
 async fn main() -> Result<(), String> {
     let args: Vec<String> = env::args().collect();

examples/get_links.rs

@@ -7,6 +7,10 @@ use netlink_packet_route::{
 };
 use rtnetlink::{new_connection, Error, Handle};
 
+#[cfg(target_os = "freebsd")]
+fn main() -> () {}
+
+#[cfg(not(target_os = "freebsd"))]
 #[tokio::main]
 async fn main() -> Result<(), ()> {
     env_logger::init();
@@ -90,6 +94,7 @@ async fn dump_links(handle: Handle) -> Result<(), Error> {
     Ok(())
 }
 
+#[cfg(not(target_os = "freebsd"))]
 async fn dump_bridge_filter_info(handle: Handle) -> Result<(), Error> {
     let mut links = handle
         .link()

examples/get_links_async.rs

@@ -7,6 +7,10 @@ use netlink_packet_route::{
 };
 use rtnetlink::{new_connection, Error, Handle};
 
+#[cfg(target_os = "freebsd")]
+fn main() -> () {}
+
+#[cfg(not(target_os = "freebsd"))]
 #[async_std::main]
 async fn main() -> Result<(), ()> {
     env_logger::init();
@@ -90,6 +94,7 @@ async fn dump_links(handle: Handle) -> Result<(), Error> {
     Ok(())
 }
 
+#[cfg(not(target_os = "freebsd"))]
 async fn dump_bridge_filter_info(handle: Handle) -> Result<(), Error> {
     let mut links = handle
         .link()

examples/get_links_thread_builder.rs

@@ -5,8 +5,11 @@ use netlink_packet_route::{
     link::{LinkAttribute, LinkExtentMask},
     AddressFamily,
 };
-use rtnetlink::{new_connection, Error, Handle};
+#[cfg(not(target_os = "freebsd"))]
+use rtnetlink::new_connection;
+use rtnetlink::{Error, Handle};
 
+#[cfg(not(target_os = "freebsd"))]
 async fn do_it(rt: &tokio::runtime::Runtime) -> Result<(), ()> {
     env_logger::init();
     let (connection, handle, _) = new_connection().unwrap();
@@ -89,6 +92,7 @@ async fn dump_links(handle: Handle) -> Result<(), Error> {
     Ok(())
 }
 
+#[cfg(not(target_os = "freebsd"))]
 async fn dump_bridge_filter_info(handle: Handle) -> Result<(), Error> {
     let mut links = handle
         .link()
@@ -109,6 +113,10 @@ async fn dump_bridge_filter_info(handle: Handle) -> Result<(), Error> {
     Ok(())
 }
 
+#[cfg(target_os = "freebsd")]
+fn main() -> () {}
+
+#[cfg(not(target_os = "freebsd"))]
 fn main() -> Result<(), String> {
     let rt = tokio::runtime::Builder::new_multi_thread()
         .enable_io()