config: prepare for build-system backend

When acting as a backend for a build-system, we should not build
any application that runs on the target, that is:
- no native gdb
- no companion libraries
- no binutils libraries
- no debug tools (save for gdbserver)
- ...

Here, we simply prepare the (hidden) config option that will detect
that we are acting as a back-end.

Update doc accordingly.

#!/bin/sh

canonicalizedname=$(readlink -nm "${0}")

dirname="$(dirname "${canonicalizedname}")"

basename="$(basename "${canonicalizedname}")"

ld_lib_path="$(dirname "${dirname}")/lib"

case ":${LD_LIBRARY_PATH}:" in

    *":${ld_lib_path}:"*)   ;;

    *)  LD_LIBRARY_PATH="${ld_lib_path}${LD_LIBRARY_PATH:+:${LD_LIBRARY_PATH}}";;

esac

export LD_LIBRARY_PATH

exec "${dirname}/.${basename}" "$@"