Accept GHCRTS=-N1 when not compiled with -threaded
> ghc Main.hs -o test > ./test +RTS -N1 test: the flag -N1 requires the program to be built with -threaded [blah blah]
> ghc Main.hs -o test > ./test +RTS -N1 hello world
Motivation: In certain environments, threaded is bad idea due to resource constraints even when multiple capabilities are available, because resident memory can increase with parallel execution. Slapping GHCRTS=-N1 onto those environments would be nice, but breaks for things not compiled with -threaded.