The build dependencies in the .cabal file are broken because of a cabal 
bug/misfeature: there are dependencies for pieces like the unit tests 
and the witnesses build that aren't built by default, which leak into 
the dependencies of the main darcs executable and library.

The fix is to make sure that everything that is conditionally buildable 
also has the rest of its section under the same condition (e.g.

if foo then
  Buildable: true
  build-depends: ...
  ...
else
  Buildable: false

I think I inadvertently made this problem worse when adding new 
dependencies to the witnesses build recently.

I intend to fix this soon, I'm just reporting the bug to make sure it's 
not forgotten and because it helps with the admin of getting the change 
into 2.5 anyway.

The following patch updated issue issue1893 with status=resolved;resolvedin=2.6.0 HEAD

* resolve issue1893: move fields of conditional builds within scope of condition 
Ignore-this: 5eb1c376138534dd55190e06be701588
This helps to work around a bug/misfeature in Cabal where it collects up
things like build-depends without looking at whether the thing they apply
to is actually buildable

The following patch updated issue issue1893 with status=resolved;resolvedin=2.5.0 CURRENT

* resolve issue1893: move fields of conditional builds within scope of condition 
Ignore-this: 5eb1c376138534dd55190e06be701588
This helps to work around a bug/misfeature in Cabal where it collects up
things like build-depends without looking at whether the thing they apply
to is actually buildable