options: Add a --clang-version flag that prints the clang version and exits.

Fixes #2138

Author: emilio

src/options.rs

@@ -28,8 +28,7 @@ where
         .usage("bindgen [FLAGS] [OPTIONS] <header> -- <clang-args>...")
         .args(&[
             Arg::with_name("header")
-                .help("C or C++ header file")
-                .required(true),
+                .help("C or C++ header file"),
             Arg::with_name("depfile")
                 .long("depfile")
                 .takes_value(true)
@@ -534,11 +533,19 @@ where
             Arg::with_name("explicit-padding")
                 .long("explicit-padding")
                 .help("Always output explicit padding fields."),
+            Arg::with_name("clang-version")
+                .long("clang-version")
+                .help("Prints the clang version, and exits"),
         ]) // .args()
         .get_matches_from(args);
 
     let mut builder = builder();
 
+    if matches.is_present("clang-version") {
+        println!("{}", crate::clang_version().full);
+        std::process::exit(0);
+    }
+
     if let Some(header) = matches.value_of("header") {
         builder = builder.header(header);
     } else {