vhost: add build.rs file to fix docs.rs build

When using `vhost` crate as a dependency, (e.g. in vhost-user-backend),
RUSTDOC_disable_feature_compat_errors is not set for the docs.rs build.
We must detect it in the build script using DOCS_RS env var.

Otherwise, building vhost-user-backend in docs.rs fails with:

  error: Both `postcopy` and `xen` features can not be enabled at the same time.
    --> vhost/src/lib.rs:63:1
     |
  63 | compile_error!("Both `postcopy` and `xen` features can not be enabled at the same time.");
     | ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

Also add the same build.rs file for vhost-user-backend since it uses the
same cfg.

Signed-off-by: Manos Pitsidianakis <manos.pitsidianakis@linaro.org>

Author: epilys

vhost-user-backend/build.rs

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
+
+fn main() {
+    println!("cargo::rerun-if-env-changed=DOCS_RS");
+    if std::env::var("DOCS_RS").is_ok() {
+        println!("cargo::rustc-cfg=RUSTDOC_disable_feature_compat_errors");
+    }
+}

vhost/build.rs

@@ -0,0 +1,8 @@
+// SPDX-License-Identifier: Apache-2.0 or BSD-3-Clause
+
+fn main() {
+    println!("cargo::rerun-if-env-changed=DOCS_RS");
+    if std::env::var("DOCS_RS").is_ok() {
+        println!("cargo::rustc-cfg=RUSTDOC_disable_feature_compat_errors");
+    }
+}