Version build dependency on STM, and allow building without it, which disables the watch command.

doc/bugs/error_building_git-annex_3.20120624_using_cabal.mdwn

@@ -152,3 +152,8 @@ I am trying to install git-annex 3.20120624 using cabal.  My currently installed
     ExitFailure 1
 
 This is using haskell-platform 2012.1.0.0~debian1 on Ubuntu 12.04.
+
+> Turns out it needs version 2.3 of the STM library. (libghc-stm-dev
+> package). I've made cabal detect an older version and skip building
+> the new `git annex watch` command, so you'll be able to build the next
+> release. [[done]] --[[Joey]]

doc/install.mdwn

@@ -37,12 +37,13 @@ To build and use git-annex, you will need:
   * [TestPack](http://hackage.haskell.org/cgi-bin/hackage-scripts/package/testpack)
   * [QuickCheck 2](http://hackage.haskell.org/package/QuickCheck)
   * [HTTP](http://hackage.haskell.org/package/HTTP)
-  * [hS3](http://hackage.haskell.org/package/hS3) (optional)
   * [json](http://hackage.haskell.org/package/json)
   * [IfElse](http://hackage.haskell.org/package/IfElse)
   * [bloomfilter](http://hackage.haskell.org/package/bloomfilter)
   * [edit-distance](http://hackage.haskell.org/package/edit-distance)
+  * [hS3](http://hackage.haskell.org/package/hS3) (optional)
   * [stm](http://hackage.haskell.org/package/stm)
+    (optional; version 2.3 or newer)
   * [hinotify](http://hackage.haskell.org/package/hinotify)
     (optional; Linux only)
 * Shell commands