Fix building cabal-install with ghc-6.12 and older
Fix building cabal-install with ghc-6.12 and older

Fall back to using serial rather job control for base < 4.3.
So this means if you build caba-install with ghc-6.12 or older
then the -j flag will do nothing, it'll still run serially.

BTW, if anyone wants to build cabal-install using a Haskell impl
with no support for concurrency then they can use the same trick.
(The serial and parallel job control impls deliberately share the
same interface.)

trskop/cabal,plumlife/cabal,thomie/cabal,typelead/epm,edsko/cabal,gridaphobe/cabal,kosmikus/cabal,anton-dessiatov/cabal,corngood/cabal,Peaker/cabal,sopvop/cabal,trskop/cabal,lukexi/cabal,edsko/cabal,rimmington/cabal,valderman/cabal,bennofs/cabal,headprogrammingczar/cabal,randen/cabal,martinvlk/cabal,Peaker/cabal,plow-technologies/cabal,ghc-android/cabal,rimmington/cabal,mydaum/cabal,Heather/cabal,kolmodin/cabal,gbaz/cabal,eric-stanley/cabal,headprogrammingczar/cabal,gridaphobe/cabal,thoughtpolice/cabal,themoritz/cabal,eric-stanley/cabal,beni55/cabal,valderman/cabal,lukexi/cabal,fpco/cabal,Helkafen/cabal,garetxe/cabal,christiaanb/cabal,thoughtpolice/cabal,fugyk/cabal,sopvop/cabal,triplepointfive/cabal,christiaanb/cabal,seereason/cabal,randen/cabal,Heather/cabal,kolmodin/cabal,fugyk/cabal,Helkafen/cabal,typelead/epm,enolan/cabal,themoritz/cabal,sopvop/cabal,x-y-z/cabal,corngood/cabal,x-y-z/cabal,mightybyte/cabal,mightybyte/cabal,martinvlk/cabal,mydaum/cabal,fpco/cabal,ghc-android/cabal,thomie/cabal,garetxe/cabal,beni55/cabal,plumlife/cabal,gbaz/cabal,kosmikus/cabal,ian-ross/cabal,plow-technologies/cabal,enolan/cabal,mmakowski/cabal,mmakowski/cabal,bennofs/cabal,themoritz/cabal,seereason/cabal,anton-dessiatov/cabal,triplepointfive/cabal,ian-ross/cabal