cabal: enable multi-threaded builds with GHC 7.8.x or later for execu…

…tables

Multi-threaded builds exacerbate the non-determinism in ghc package ids, which
is a serious problem for libraries. Packages that define only executables,
however, should be safe to  build with parallelism enabled.

pkgs/build-support/cabal/default.nix

@@ -217,6 +217,9 @@ assert !enableStaticLibraries -> versionOlder "7.7" ghc.version;
               ${optionalString (self.enableSharedExecutables && self.stdenv.isDarwin) ''
                 configureFlags+=" --ghc-option=-optl=-Wl,-headerpad_max_install_names"
               ''}
+              ${optionalString (versionOlder "7.8" ghc.version && !self.isLibrary) ''
+                configureFlags+=" --ghc-option=-j$NIX_BUILD_CORES"
+              ''}
 
               echo "configure flags: $extraConfigureFlags $configureFlags"
               ./Setup configure --verbose --prefix="$out" --libdir='$prefix/lib/$compiler' \