pkgs/build-support/cabal: move current postFixup hook into installPhase to free up the postFixup hook for users to override

Builds tend to override pre and post hooks for each phase, so we should not use
them to perform regular build commands that cannot be omitted.
pkgs/build-support/cabal: stabilize shared linking support

We cannot pass the --{enable,disable}-executable-dynamic flags to GHC
versions prior to 7.4.x.

Building shared libraries via --{enable,disable}-shared is possible in theory
with GHC 6.12.x or later, but doesn't work in practice because our GHC 6.10.x
builds don't provide shared versions of their base libraries. This could
probably be fixed, but it's probably not worth the effort.
ghc-wrapper: rename 'ghcVersion' attribute to 'version' for consistency