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.