[Hackage] #312: Allow ${arch} var in install paths

[Hackage]

#312: Allow ${arch} var in install paths
----------------------------+-----------------------------------------------
  Reporter:  larsv          |        Owner:                     
      Type:  enhancement    |       Status:  new                
  Priority:  normal         |    Milestone:  Cabal-1.4          
 Component:  Cabal library  |      Version:  1.2.3.0            
  Severity:  minor          |   Resolution:                     
  Keywords:                 |   Difficulty:  very easy (<1 hour)
Ghcversion:  6.8.2          |     Platform:                     
----------------------------+-----------------------------------------------
Changes (by duncan):

  * difficulty:  easy (<4 hours) => very easy (<1 hour)
  * summary:  Multiple architectures under .cabal/ => Allow ${arch} var in
              install paths
  * milestone:  => Cabal-1.4

Comment:

 This is just a matter of adding an `ArchVar` constructor to
 `PathTemplateVariable` in the `Distribution.Simple.InstallDirs` module.
 Then the `Arch` should be passed to `substituteTemplates` along with the
 `CompilerId` etc.

 This will make it possible for users to use `${arch}` in the install dirs
 like `--libdir` (or the equivalents in the cabal-install config file).
 Note that we're not proposing to actually change the default install
 paths, just to make it easy for users to do this themselves.

-- 
Ticket URL: <http://hackage.haskell.org/trac/hackage/ticket/312#comment:1>
Hackage <http://haskell.org/cabal/>
Hackage: Cabal and related projects