anvil: properly document logind support

README.md

@@ -32,7 +32,9 @@ Like others, Smithay as a compositor library has its own sample compositor: anvi
 You can run it with cargo after having cloned this repository:
 
 ```
-cargo run -p anvil -- --{backend}
+cd anvil;
+
+cargo run -- --{backend}
 ```
 
 The currently available backends are:
@@ -41,4 +43,9 @@ The currently available backends are:
   inside of an other X11 or Wayland session.
 - `--tty-udev`: start anvil in a tty with udev support. This is the "traditional" launch of a Wayland
   compositor. Note that this requires you to start anvil as root if your system does not have logind
-  available ([consolekit support is planned](https://github.com/Smithay/smithay/issues/95)).
+  available ([consolekit support is planned](https://github.com/Smithay/smithay/issues/95)). To use logind,
+  you need to activate the associated cargo feature:
+
+  ```
+  cargo run --features logind -- --tty-udev
+  ```

anvil/build.rs

@@ -0,0 +1,10 @@
+use std::env::var;
+
+fn main() {
+    if !var("CARGO_FEATURE_LOGIND").ok().is_some() {
+        println!("cargo:warning=You are compiling anvil without logind support.");
+        println!("cargo:warning=This means that you'll likely need to run it as root if you want to launch it from a tty.");
+        println!("cargo:warning=To enable logind support add `--feature logind` to your cargo invocation:");
+        println!("cargo:warning=$ cd anvil; cargo run --feature logind");
+    }
+}