explicit deps for custom setup script

[bbarker]

From version 1.24 cabal supports specifying explicit dependencies for Custom setup scripts. Consider using cabal-version >= 1.24 and adding a 'custom-setup' section with a 'setup-depends' field that specifies the dependencies of the Setup.hs script itself. The 'setup-depends' field uses the same syntax as 'build-depends', so a simple example would be 'setup-depends: base, Cabal'.