Add custom CC wrapper to the wrapper whitelist

Some toolchains use custom (or at least not widely used) CC wrappers,
and passes that through the CC env var before the compiler, like:

  CC = <custom_wrapper> <something>-gcc

Add support to CC_KNOWN_WRAPPER_CUSTOM env var, to be able to extend the
'known_wrappers' list with this <custom_wrapper>, and handle these
special cases.

src/lib.rs

@@ -3120,7 +3120,11 @@ impl Build {
         //
         // It's true that everything here is a bit of a pain, but apparently if
         // you're not literally make or bash then you get a lot of bug reports.
-        let known_wrappers = ["ccache", "distcc", "sccache", "icecc", "cachepot"];
+        let mut known_wrappers = vec!["ccache", "distcc", "sccache", "icecc", "cachepot"];
+        let custom_wrapper = self.getenv("CC_KNOWN_WRAPPER_CUSTOM");
+        if custom_wrapper.is_some() {
+            known_wrappers.push(custom_wrapper.as_deref().unwrap().to_str().unwrap());
+        }
 
         let mut parts = tool.split_whitespace();
         let maybe_wrapper = match parts.next() {