Add -threaded and -rtsopts to cabal-install's ghc-options.

We don't want FFI calls to block all Haskell threads, so we need to use the
threaded runtime; -rtsopts is for experimenting with the number of capabilities.

cabal-install/cabal-install.cabal

@@ -38,7 +38,7 @@ flag bytestring-in-base
 
 Executable cabal
     Main-Is:            Main.hs
-    ghc-options:        -Wall
+    ghc-options:        -Wall -threaded -rtsopts
     if impl(ghc >= 6.8)
       ghc-options: -fwarn-tabs
     Other-Modules: