workcache: Don't assume gcc exists on all platforms

FreeBSD has recently moved to clang by default, and no longer ship gcc. Instead
use "cc" on unix platforms (the default compiler) and "gcc" on windows.

src/libworkcache/lib.rs

@@ -512,8 +512,9 @@ fn test() {
         prep.declare_input("file", pth.as_str().unwrap(), file_content);
         prep.exec(proc(_exe) {
             let out = make_path(~"foo.o");
+            let compiler = if cfg!(windows) {"gcc"} else {"cc"};
             // FIXME (#9639): This needs to handle non-utf8 paths
-            Process::status("gcc", [pth.as_str().unwrap().to_owned(),
+            Process::status(compiler, [pth.as_str().unwrap().to_owned(),
                                     ~"-o",
                                     out.as_str().unwrap().to_owned()]).unwrap();