Cabal.cabal: change build-type to Simple.

Fixes #3003.

Cabal/Cabal.cabal

@@ -19,7 +19,7 @@ description:
   organizing, and cataloging Haskell libraries and tools.
 category: Distribution
 cabal-version: >=1.10
-build-type: Custom
+build-type: Simple
 -- Even though we do use the default Setup.lhs it's vital to bootstrapping
 -- that we build Setup.lhs using our own local Cabal source code.