make git optional to support offline and packaged builds

Previously the build script unconditionally required `git` to update
submodules. This breaks offline builds for vendored workspaces and
environments without git installed where the files are already present
but `.git` is absent.

The new build flow is as follows:

- Conditionally run `git submodule` only if the `mavlink` directory is a
  submodule.
- Add an explicit existence check for the `message_definitions/v1.0`
  directory to provide a clear error message if submodule was omitted.

Signed-off-by: Onur Özkan <work@onurozkan.dev>

Author: onur-ozkan

mavlink/build/main.rs

@@ -9,30 +9,30 @@ use mavlink_bindgen::XmlDefinitions;
 
 fn main() -> ExitCode {
     let src_dir = Path::new(env!("CARGO_MANIFEST_DIR"));
+    let mavlink_dir = src_dir.join("mavlink");
 
-    // Check if git is installed
-    if Command::new("git").arg("--version").status().is_err() {
-        eprintln!("error: Git is not installed or could not be found.");
-        return ExitCode::FAILURE;
-    }
-
-    // Update and init submodule
-    if let Err(error) = Command::new("git")
-        .arg("submodule")
-        .arg("update")
-        .arg("--init")
-        .current_dir(src_dir)
-        .status()
-    {
-        eprintln!("Failed to update MAVLink definitions submodule: {error}");
-        return ExitCode::FAILURE;
+    // It is a submodule if it contains `.git` or if it's completely empty (uninitialized)
+    let is_mavlink_empty = read_dir(&mavlink_dir)
+        .map(|mut d| d.next().is_none())
+        .unwrap_or(true);
+    let is_submodule = mavlink_dir.join(".git").exists() || is_mavlink_empty;
+
+    if is_submodule {
+        if let Err(error) = Command::new("git")
+            .arg("submodule")
+            .arg("update")
+            .arg("--init")
+            .current_dir(src_dir)
+            .status()
+        {
+            eprintln!("Failed to update MAVLink definitions submodule: {error}");
+            return ExitCode::FAILURE;
+        }
     }
 
     // find & apply patches to XML definitions to avoid crashes
     let patch_dir = src_dir.join("build/patches");
-    let mavlink_dir = src_dir.join("mavlink");
-
-    if let Ok(dir) = read_dir(patch_dir) {
+    if let Ok(dir) = read_dir(&patch_dir) {
         for entry in dir.flatten() {
             if let Err(error) = Command::new("git")
                 .arg("apply")
@@ -46,9 +46,17 @@ fn main() -> ExitCode {
         }
     }
 
-    let out_dir = env::var("OUT_DIR").unwrap();
+    let source_definitions_dir = mavlink_dir.join("message_definitions/v1.0");
 
-    let source_definitions_dir = src_dir.join("mavlink/message_definitions/v1.0");
+    // Check if the source definitions directory exists
+    if !source_definitions_dir.is_dir() {
+        eprintln!(
+            "MAVLink message definitions directory not found at: {}\n\
+             Ensure submodules are included.",
+            source_definitions_dir.display(),
+        );
+        return ExitCode::FAILURE;
+    }
 
     let enabled_dialects: Vec<String> = env::vars()
         .filter_map(|(key, _)| {
@@ -59,19 +67,19 @@ fn main() -> ExitCode {
 
     let mut definitions_to_bind = vec![];
 
-    if let Ok(dir) = read_dir(&source_definitions_dir) {
-        for entry in dir.flatten() {
-            let filename = entry
-                .path()
-                .file_stem()
-                .unwrap()
-                .to_string_lossy()
-                .to_lowercase();
-
-            if enabled_dialects.contains(&filename) {
-                definitions_to_bind.push(entry.path());
-            }
+    // Check if the expected dialects requested by Cargo features are missing
+    for dialect in &enabled_dialects {
+        let dialect_file = source_definitions_dir.join(format!("{dialect}.xml"));
+
+        if !dialect_file.is_file() {
+            eprintln!(
+                "Expected MAVLink dialect definition not found at: {}",
+                dialect_file.display(),
+            );
+            return ExitCode::FAILURE;
         }
+
+        definitions_to_bind.push(dialect_file);
     }
 
     let xml_definitions = if definitions_to_bind.is_empty() {
@@ -80,6 +88,7 @@ fn main() -> ExitCode {
         XmlDefinitions::Files(definitions_to_bind)
     };
 
+    let out_dir = env::var("OUT_DIR").unwrap();
     let result = match mavlink_bindgen::generate(xml_definitions, out_dir) {
         Ok(r) => r,
         Err(e) => {