/usr/local hard-coded

This is a migration. GHC Trac user 'donn' reports in https://ghc.haskell.org/trac/ghc/ticket/5827:
- libraries/Cabal/Cabal/Distribution/Simple/InstallDirs.hs:defaultInstallDirs returns "/usr/local"
- and in same directory, Hugs.hs packageDbPaths includes "/usr/local/lib/hugs/packages"
- and cabal-install/bootstrap.sh forces PREFIX=/usr/local/.
- ... and, I don't think it actually matters? but also in Cabal/Makefile.

"/usr/local" ordinarily does not exist on the platform I'm building for (Haiku), so it isn't a convenient default for cabal. The `prefix' value from ghc build configure would work for this case.