parinfer is not built with -threaded

[ehigham]

In the "Parallel Type Inferencer" example in chapter 4, parinfer is invoked like this

$ ./parinfer <benchmark.in +RTS -s -N2

The recipe for parinfer in parconc-examples.cabal doesn't allow this, however, as parinfer is not built with -threaded. Running with stack following a stack build leads to the following error:

$ stack exec parinfer <parinfer/benchmark.in -- +RTS -s -N2
parinfer: the flag -N2 requires the program to be built with -threaded