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)

configure.ac

@@ -153,6 +153,9 @@ AC_SUBST(storedir)
 # and CPPFLAGS are not passed to the C++ compiler automatically.
 # Thus we append the returned CPPFLAGS to the CXXFLAGS here.
 AX_BOOST_BASE([1.66], [CXXFLAGS="$BOOST_CPPFLAGS $CXXFLAGS"], [AC_MSG_ERROR([Nix requires boost.])])
+# For unknown reasons, setting this directly in the ACTION-IF-FOUND above
+# ends up with LDFLAGS being empty, so we set it afterwards.
+LDFLAGS="$BOOST_LDFLAGS $LDFLAGS"
 
 
 # Look for OpenSSL, a required dependency.