nix-store -l: Fetch build logs from the Internet

If a build log is not available locally, then ‘nix-store -l’ will now
try to download it from the servers listed in the ‘log-servers’ option
in nix.conf. For instance, if you have:

  log-servers = http://hydra.nixos.org/log

then it will try to get logs from http://hydra.nixos.org/log/<base
name of the store path>. So you can do things like:

  $ nix-store -l $(which xterm)

and get a log even if xterm wasn't built locally.

Makefile.config.in

@@ -10,6 +10,7 @@ PACKAGE_VERSION = @PACKAGE_VERSION@
 bash = @bash@
 bindir = @bindir@
 bsddiff_compat_include = @bsddiff_compat_include@
+curl = @curl@
 datadir = @datadir@
 datarootdir = @datarootdir@
 dblatex = @dblatex@

doc/manual/conf-file.xml

@@ -465,6 +465,20 @@ flag, e.g. <literal>--option gc-keep-outputs false</literal>.</para>
   </varlistentry>
 
 
+  <varlistentry xml:id="conf-log-servers"><term><literal>log-servers</literal></term>
+
+    <listitem>
+
+      <para>A list of URL prefixes (such as
+      <literal>http://hydra.nixos.org/log</literal>) from which
+      <command>nix-store -l</command> will try to fetch build logs if
+      they’re not available locally.</para>
+
+    </listitem>
+
+  </varlistentry>
+
+
 </variablelist>
 
 </para>