Only build iserv with -threaded if GhcThreaded is set
Only build iserv with -threaded if GhcThreaded is set

By default GhcThreaded is set by
```
GhcThreaded = $(if $(findstring thr,$(GhcRTSWays)),YES,NO)
```
so it seems incorrect to try to build iserv with -threaded when
GhcThreaded is not set. This came up when I was building GHC with
some strange combination of flavours (debugged and profiled but not
threaded).

Test Plan: harbormaster

Reviewers: simonmar, austin, bgamari

Reviewed By: bgamari

Subscribers: thomie, snowleopard

Differential Revision: https://phabricator.haskell.org/D3429

ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,sdiehl/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,ezyang/ghc,ezyang/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,sdiehl/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc,ezyang/ghc,shlevy/ghc,shlevy/ghc,sdiehl/ghc