pkgs/build-support/cabal: the configure-phase warning check can be

disabled by setting 'strictConfigurePhase' to 'false'

This is necessary for some packages, like dns, because cabal warns about
multiple versions of the same dependency being used, but the usage is fine,
actually, so we want the build to succeed. Packages that depend on 'doctest'
also have this issue <https://github.com/sol/doctest-haskell/issues/69>.