Fix #45345.

Re-implement

```bash
CFG_LIBDIR_RELATIVE=`echo ${CFG_LIBDIR} | cut -c$((${#CFG_PREFIX}+${CAT_INC}))-`
```

from old `configure` script.

Author: o01eg

src/bootstrap/config.rs

@@ -403,6 +403,7 @@ impl Config {
             config.docdir = install.docdir.clone().map(PathBuf::from);
             config.bindir = install.bindir.clone().map(PathBuf::from);
             config.libdir = install.libdir.clone().map(PathBuf::from);
+            config.libdir_relative = install.libdir.clone().map(PathBuf::from);
             config.mandir = install.mandir.clone().map(PathBuf::from);
         }