libtool is adding "-L/usr/lib" even for cross-compilation with sysroot

Hi libtool dev,

We've recently hit an issue cross-compiling some package using libtool
for sysroot.  Saying libfoo.la contains

libdir=/usr/lib

Then libtool will add "-L/usr/lib" to the linker command line during a
"relink" against libfoo.la, because ltmain.in:6500:

> 	    else
> 	      # We cannot seem to hardcode it, guess we'll fake it.
> 	      add_dir=-L$libdir
                      ^^^^^^^^^
> 	      # Try looking first in the location we're being installed to.
> 	      if test -n "$inst_prefix_dir"; then
> 		case $libdir in
> 		  [\\/]*)
> 		    func_append add_dir " -L$inst_prefix_dir$libdir"
> 		    ;;
> 		esac
> 	      fi
> 	      add=-l$name
> 	    fi

Now the linker will try to use the system library in /usr/lib, causing
link error or runtime issue.

There is a patch at
https://lists.gnu.org/archive/html/libtool-patches/2021-10/msg00017.html
changing -L$libdir to -L$lt_sysroot$libdir at this line, but maybe the
author didn't submitted it properly (I guess it should be submitted into
https://savannah.gnu.org/patch/?group=libtool, instead of the list). 
Could this change, or something similar be applied in the incoming new
release?
-- 
Xi Ruoyao <[email protected]>
School of Aerospace Science and Technology, Xidian University