Get BOOST_LDFLAGS from autoconf, fix Ubuntu 16.04 build.

Our use of boost::coroutine2 depends on -lboost_context,
which in turn depends on `-lboost_thread`, which in turn depends
on `-lboost_system`.

I suspect that this builds on nix only because of low-level hacks
like NIX_LDFLAGS.

This commit passes the proper linker flags, thus fixing bootstrap
builds on non-nix distributions like Ubuntu 16.04.

With these changes, I can build Nix on Ubuntu 16.04 using:

    ./bootstrap.sh
    ./configure --prefix=$HOME/editline-prefix \
      --disable-doc-gen \
      CXX=g++-7 \
      --with-boost=$HOME/boost-prefix \
      EDITLINE_CFLAGS=-I$HOME/editline-prefix/include \
      EDITLINE_LIBS=-leditline \
      LDFLAGS=-L$HOME/editline-prefix/lib
    make

where

* g++-7 comes from gcc-7 from
  https://launchpad.net/~ubuntu-toolchain-r/+archive/ubuntu/test,
* editline 1.14 from https://github.com/troglobit/editline/releases/tag/1.14.0
	was installed into `$HOME/editline-prefix`
  (because Ubuntu 16.04's `editline` is too old to have the function nix uses),
* boost 1.66 from
	https://www.boost.org/doc/libs/1_66_0/more/getting_started/unix-variants.html
	was installed into $HOME/boost-prefix (because Ubuntu 16.04 only has 1.58)

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