This might not work as it should.
"if test ! -d ${with_top_level}/usr; then"
this line does not what it should and here is why:
I have found that when --with-sysroot is given, the places to look for libraries are these: $sysroot/include;$sysroot/lib
I would have to give --with-sysroot=$realsysroot/usr to get libraries in $realsysroot/usr. Now, obviously $realsysroot/usr/usr does not exist.
When I give --with-sysroot=$realsysroot, no libs are found. When I give --with-sysroot=$realsysroot/usr, the path check is triggered and configure fails.

when giving configure the --with-sysroot=/some/path, my /some/path is lost. From my research, the configure script keeps a variable $incllist. To this it appends the path given as sysroot. On line 525 it starts to check if directories are valid.
[5 minutes later]
I now discovered that I had a typo in my path and it didnt eexist. Still, maybe its a bad idea to silently drop a path. should the --with-sysroot value be checked for existence earlier to report an error when that dir doesnt exist?