haskell-cabal.eclass: fix --sysconfdir for cabal-1.16 and lower

The option introduced in dd7a21545b30372c78fe7247c564f10c22016a43
does not work for cases when we use Cabal bundled with ghc
(haskell-updater, hsqm).

Added simple runtime detection support.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>