pkgs/build-support/cabal: add 'enableSplitObjs' field to enable/disable object splitting

This patch configures all Cabal builds with '--enable-split-objs' unless the
Nix expression explicitly sets "enableSplitObjs = false". The Cabal manual [1]
describes this option as follows:

 | The GHC -split-objs reduces the final size of the executables that use the
 | library by allowing them to link with only the bits that they use rather
 | than the entire library. The downside is that building the library takes
 | longer and uses considerably more memory.

One immediate benefit of this change is that the 'darcs' closure defined in the
top-level no longer refers to GHC. The same is probably true with other
executable packages.

[1] http://www.haskell.org/cabal/users-guide/installing-packages.html#setup-configure