Default to llvm-ar when compiling with clang.

Defaulting to llvm-ar facilitates cases when system ar(1) doesn't
support any given cross-compile target. This is because accompanying
llvm-ar is known to match targets supported by the clang installation
at hand.

src/lib.rs

@@ -1016,7 +1016,7 @@ impl Build {
             None => "none",
         };
         if cudart != "none" {
-            if let Some(nvcc) = which(&self.get_compiler().path) {
+            if let Some(nvcc) = which(&self.get_compiler().path, None) {
                 // Try to figure out the -L search path. If it fails,
                 // it's on user to specify one by passing it through
                 // RUSTFLAGS environment variable.
@@ -2496,7 +2496,15 @@ impl Build {
                 None => default_ar,
             }
         } else {
-            default_ar
+            let compiler = self.get_base_compiler()?;
+            if compiler.family == ToolFamily::Clang {
+                match search_programs(&mut self.cmd(&compiler.path), "llvm-ar") {
+                    Some(ar) => ar.to_str().unwrap().to_owned(),
+                    None => default_ar,
+                }
+            } else {
+                default_ar
+            }
         };
         Ok((self.cmd(&program), program))
     }
@@ -3242,7 +3250,7 @@ fn map_darwin_target_from_rust_to_compiler_architecture(target: &str) -> Option<
     }
 }
 
-fn which(tool: &Path) -> Option<PathBuf> {
+fn which(tool: &Path, path_entries: Option<OsString>) -> Option<PathBuf> {
     fn check_exe(exe: &mut PathBuf) -> bool {
         let exe_ext = std::env::consts::EXE_EXTENSION;
         exe.exists() || (!exe_ext.is_empty() && exe.set_extension(exe_ext) && exe.exists())
@@ -3255,9 +3263,23 @@ fn which(tool: &Path) -> Option<PathBuf> {
     }
 
     // Loop through PATH entries searching for the |tool|.
-    let path_entries = env::var_os("PATH")?;
+    let path_entries = path_entries.or(env::var_os("PATH"))?;
     env::split_paths(&path_entries).find_map(|path_entry| {
         let mut exe = path_entry.join(tool);
         return if check_exe(&mut exe) { Some(exe) } else { None };
     })
 }
+
+// search for |prog| on 'programs' path in '|cc| -print-search-dirs' output
+fn search_programs(cc: &mut Command, prog: &str) -> Option<PathBuf> {
+    let search_dirs = run_output(cc.arg("-print-search-dirs"), "cc").ok()?;
+    // clang driver appears to be forcing UTF-8 output even on Windows,
+    // hence from_utf8 is assumed to be usable in all cases.
+    let search_dirs = std::str::from_utf8(&search_dirs).ok()?;
+    for dirs in search_dirs.split(|c| c == '\r' || c == '\n') {
+        if dirs.starts_with("programs: =") {
+            return which(Path::new(prog), Some(OsString::from(&dirs[11..])));
+        }
+    }
+    None
+}