Remove warning on option -threaded when building libraries (#8432)

* Remove warning on option -threaded when building libraries

Fixes #8431. Accompanied by option -flink-rts, option -threaded defines
the flavour of the ghc RTS library the built library will be linked
against. Note that bare ghc does not warn when option -threaded is used
for building a library either.

* Cabal no longer warns on -threaded option when building libraries

Note that this commit modifies regression check for issue #774.

* A small changelog was added