move nix files to separate dir

contrib/nix/taler-exchange-dev.nix

@@ -0,0 +1,61 @@
+{ stdenv, makeWrapper, pkgconfig, autoconf, automake, libtool, ccache, ccache_dir ? ""
+, gnunet-dev, postgresql, jansson, libgcrypt, libgnurl, libmicrohttpd }:
+
+stdenv.mkDerivation rec {
+  src = ./.;
+  name = "taler-exchange-dev";
+
+  buildInputs = [
+    makeWrapper pkgconfig autoconf automake libtool ccache
+    gnunet-dev postgresql jansson libgcrypt libgnurl libmicrohttpd
+  ];
+
+  patchPhase = ''
+    if [ -e Makefile ]; then
+      make distclean
+    fi
+  '';
+
+  NIX_CFLAGS_COMPILE = "-ggdb -O0";
+
+  configureFlags = [
+    "--enable-gcc-hardening"
+    "--enable-linker-hardening"
+
+    "--enable-logging=verbose"
+    "--enable-poisoning"
+  ];
+
+  preConfigure = ''
+    ./bootstrap
+
+    if [ -n "${ccache_dir}" ]; then
+      export CC='ccache gcc'
+      export CCACHE_COMPRESS=1
+      export CCACHE_DIR="${ccache_dir}"
+      export CCACHE_UMASK=007
+    fi
+  '';
+
+  doCheck = false;
+
+  postInstall = ''
+    # Tests can be run this way
+    #export GNUNET_PREFIX="$out"
+    #export PATH="$out/bin:$PATH"
+    #make -k check
+  '';
+
+  meta = with stdenv.lib; {
+    description = "Exchange for GNU Taler";
+
+    longDescription = ''
+    '';
+
+    homepage = https://taler.net/;
+
+    license = licenses.gpl3Plus;
+    platforms = platforms.gnu;
+    maintainers = with maintainers; [ ];
+  };
+}