Only register Flight.proto with cargo if it exists (#351) (#374)

Co-authored-by: Raphael Taylor-Davies <1781103+tustvold@users.noreply.github.com>

arrow-flight/build.rs

@@ -23,16 +23,16 @@ use std::{
 };
 
 fn main() -> Result<(), Box<dyn std::error::Error>> {
-    // avoid rerunning build if the file has not changed
-    println!("cargo:rerun-if-changed=../format/Flight.proto");
-
     // override the build location, in order to check in the changes to proto files
     env::set_var("OUT_DIR", "src");
 
     // The current working directory can vary depending on how the project is being
     // built or released so we build an absolute path to the proto file
     let path = Path::new("../format/Flight.proto");
     if path.exists() {
+        // avoid rerunning build if the file has not changed
+        println!("cargo:rerun-if-changed=../format/Flight.proto");
+
         tonic_build::compile_protos("../format/Flight.proto")?;
         // read file contents to string
         let mut file = OpenOptions::new()