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)

1 files changed, 1 insertions, 1 deletions

diff --git a/src/nix/local.mk b/src/nix/local.mk
index ca4604d566c3..c09efd1fc895 100644
--- a/src/nix/local.mk
+++ b/src/nix/local.mk
@@ -17,7 +17,7 @@ nix_SOURCES := \
 
 nix_LIBS = libexpr libmain libstore libutil
 
-nix_LDFLAGS = -pthread $(SODIUM_LIBS) $(EDITLINE_LIBS)
+nix_LDFLAGS = -pthread $(SODIUM_LIBS) $(EDITLINE_LIBS) $(BOOST_LDFLAGS) -lboost_context -lboost_thread -lboost_system
 
 $(foreach name, \
   nix-build nix-channel nix-collect-garbage nix-copy-closure nix-daemon nix-env nix-hash nix-instantiate nix-prefetch-url nix-shell nix-store, \