build-sys: Recommend --with-rootprefix=/ for split-usr

Since we started using AX_NORMALIZE_PATH, that is a valid supported
setup and is more explicit than --with-rootprefix= (empty) which is
actually currently broken.

Let's advocate for it in the ./configure suggestion from autogen.sh.

autogen.sh

@@ -47,7 +47,7 @@ fi
 
 if [ ! -L /bin ]; then
 args="$args \
---with-rootprefix= \
+--with-rootprefix=/ \
 --with-rootlibdir=$(libdir /lib) \
 "
 fi