Add libname install-dirs variable, use it by default. Fixes #2437.
Add libname install-dirs variable, use it by default. Fixes #2437.

Signed-off-by: Edward Z. Yang <dbd597f5635f432486c5d365e9bb585b3eaa1853@cs.stanford.edu>

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