haskell-cabal.eclass: drop unused CABAL_FEATURES="alex happy c2hs cpphs"

haskell-cabal.eclass: drop unused CABAL_FEATURES="alex happy c2hs cpphs"

While at it:
- dropped internal 'ghc-sanecabal()' helper
- factored out parallelism setup to new 'ghc-make-args()' function
- building initial Setup binary now also uses 'ghc-make-args()'
- dropped unused CABAL_MIN_VERSION global variable

Fixed bug in 'ghc-make-args()' to work on NOSMP ports of GHC:
We use 'qb0' to enable parallel GC. Parallel GC requires SMP
support and threaded runtime.

Reported by iota__ on arm/ghc-8 where there is support for
parallel make but not the SMP support:

    ,("Host platform","arm-unknown-linux")
    ,("Support SMP","NO")
    ,("Support parallel --make","YES")
    ,("RTS ways","l debug thr...

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