[Haskell-beginners] the flag -N2 requires the program to be built with -threaded

[Jung Kim]

I am currently reading an article[1] where the author uses Haskell to
explain her idea. The problem is I don't understand Haskell and am not
familiar with Haskell concept like MVar, IVar, etc. (I can get the
code compile, and have several years programming experiences.)

When testing the code, it throws the error messages "the flag -N2
requires the program to be built with -threaded"; however I can't find
thread related flag with ghc command by `ghc --help`. How can I fix
this problem?

Environment I use: Debian stretch/sid, kernel 4.0.0-2-rt-686-pae, GHC
version 7.10.3.

The code is exactly the same as described in the section
'Nondeterminism with MVars' of article; and the makefile content is

all:
        ghc -O mvar.hs
        ghc -o mvar mvar.o
run:
        ./execute # while true; do ./mvar +RTS -N2; done
clean:
        rm mvar

Thanks

[1]. http://composition.al/blog/2013/09/22/some-example-mvar-ivar-and-lvar-programs-in-haskell/