fix: last --jobserver-auth wins

From the GNU make manual[^1]:

> Be aware that the `MAKEFLAGS` variable may contain multiple
> instances of the `--jobserver-auth=` option.
> Only the last instance is relevant.

Hence this commit makes it so.

With this commit, `--jobserver-auth` also takes precedence over
`--jobserver-fds`, even when `--jobserver-fds` is the last instance
of these flags. This is made intentionally since `--jobserver-fds`
was an undocumented internal-only flag before `--jobserver-auth`
made public, according to this announcement[^2].

[^1]: https://www.gnu.org/software/make/manual/make.html#Job-Slots
[^2]: https://lists.gnu.org/archive/html/info-gnu/2016-05/msg00013.html

Author: weihanglo

src/lib.rs

@@ -584,10 +584,23 @@ impl HelperState {
     }
 }
 
+/// Finds the position of `--jobserver-auth=` flag in a given environment variable.
+///
+/// Precedence rules:
+///
+/// * The last instance wins [^1].
+/// * `--jobserver-fds=` as a fallback when no `--jobserver-auth=` is present [^2].
+///
+/// [^1]: See ["GNU male manual: Sharing Job Slots with GNU make"](https://www.gnu.org/software/make/manual/make.html#Job-Slots)
+///   _"Be aware that the `MAKEFLAGS` variable may contain multiple instances
+///   of the `--jobserver-auth=` option. Only the last instance is relevant."_
+///
+/// [^2]: See [the release announcement](https://lists.gnu.org/archive/html/info-gnu/2016-05/msg00013.html)
+///   of GNU Make 4.2.
 fn find_jobserver_auth(var: &str) -> Option<(&str, usize)> {
-    ["--jobserver-fds=", "--jobserver-auth="]
+    ["--jobserver-auth=", "--jobserver-fds="]
         .iter()
-        .map(|&arg| var.find(arg).map(|pos| (arg, pos)))
+        .map(|&arg| var.rfind(arg).map(|pos| (arg, pos)))
         .find_map(|pos| pos)
 }
 
@@ -603,19 +616,19 @@ fn test_find_jobserver_auth() {
     let cases = [
         (
             "--jobserver-auth= --jobserver-auth=",
-            ("--jobserver-auth=", 0),
+            ("--jobserver-auth=", 18),
         ),
         (
             "--jobserver-fds= --jobserver-fds=",
-            ("--jobserver-fds=", 0usize),
+            ("--jobserver-fds=", 17),
         ),
         (
             "--jobserver-auth= --jobserver-fds= --jobserver-auth=",
-            ("--jobserver-fds=", 18),
+            ("--jobserver-auth=", 35),
         ),
         (
             "--jobserver-fds= --jobserver-auth= --jobserver-fds=",
-            ("--jobserver-fds=", 0),
+            ("--jobserver-auth=", 17),
         ),
     ];
     for (var, expected) in cases {