curl-sys: never try to link with /lib/darwin

`clang --print-search-dirs` may return `libraries: =` that leads to
`/lib/darwin` which may leads to attempt to find `clang_rt.osx` inside
this wired path and which fails the build.

vendor/curl-sys/build.rs

@@ -532,7 +532,9 @@ fn macos_link_search_path() -> Option<String> {
     for line in stdout.lines() {
         if line.contains("libraries: =") {
             let path = line.split('=').skip(1).next()?;
-            return Some(format!("{}/lib/darwin", path));
+            if !path.is_empty() {
+                return Some(format!("{}/lib/darwin", path));
+            }
         }
     }