enable parallel build for fast mode and explicitly disable for production

Currently, ghc has issues getting reproducible builds with parallel
building. https://ghc.haskell.org/trac/ghc/ticket/4012

debian/changelog

@@ -8,6 +8,9 @@ git-annex (5.20150813) UNRELEASED; urgency=medium
   * sync: Support --jobs
   * sync --content: Avoid unnecessary second pull from remotes when 
     no file transfers are made.
+  * External special remotes can now be built that can be used in readonly
+    mode, where git-annex downloads content from the remote using regular
+    http.
 
  -- Joey Hess <id@joeyh.name>  Wed, 12 Aug 2015 14:31:01 -0400
 

doc/design/external_special_remote_protocol.mdwn

@@ -343,6 +343,26 @@ quite a long time, especially when the git-annex assistant is using it.
 The assistant will detect when the system connects to a network, and will
 start a new process the next time it needs to use a remote.
 
+## readonly mode
+
+Some storage services allow downloading the content of a file using a
+regular http connection, with no authentication. An external special remote
+for such a storage service can support a readonly mode of operation.
+
+It works like this:
+
+* When a key's content is stored on the remote, use SETURLPRESENT to
+  tell git-annex the public url from which it can be downloaded.
+* When a key's content is removed from the remote, use SETURLMISSING.
+* Document that this external special remote can be used in readonly mode.
+
+  The user doesn't even need to install your external special remote
+  program to use such a remote! All they need to do is run:
+  `git annex enableremote $remotename readonly=true`
+
+* The readonly=true parameter makes git-annex download content from the
+  urls recorded earlier by SETURLPRESENT.
+
 ## TODO
 
 * When storing encrypted files stream the file up/down the pipe, rather

git-annex.cabal

@@ -152,8 +152,13 @@ Executable git-annex
   else
     Build-Depends: cryptohash (>= 0.11.0)
 
+  -- Fully optimize for production.
+  -- Parallel builds only when not building for production,
+  -- because ghc is known to not yield reproducible builds this way.
   if flag(Production)
-    GHC-Options: -O2
+    GHC-Options: -O2 -j1
+  else
+    GHC-Options: -j
 
   if (os(windows))
     Build-Depends: Win32, Win32-extras, unix-compat (>= 0.4.1.3), setenv