We must not pass '--disable-benchmarks' to setup scripts built with Cabal <
1.14; the same is also true for '--disable-tests' and Cabal < 1.10.

Fixes #932 and #1004.

Without this patch:

    $ cabal install -w /path/to/ghc-7.2.1/bin/ghc syb
    [...]
    unrecognized option `--disable-benchmarks'
    Failed to install syb-0.3.7

With this patch:

    $ cabal install -w /path/to/ghc-7.2.1/bin/ghc syb
    [...]
    Registering syb-0.3.7...
    Installed syb-0.3.7