It sucks, I know, but GHC just doesn't compile reliably when built with
some -j<n> option. :-( We have build errors because of apparent race
conditions all over the place on Hydra. This causes so much trouble for
users that it's not worth keeping this option enabled, IMHO.
I got the following error in 4 consecutive attempts:
building rts/dist/build/AutoApply.debug_o
building rts/dist/build/AutoApply.thr_o
rts_dist_HC rts/dist/build/AutoApply.debug_o
/nix/store/1iigiim5855m8j7pmwf5xrnpf705s4dh-binutils-2.21.1a/bin/ld: cannot find libraries/integer-gmp/dist-install/build/cbits/gmp-wrappers_o_split/gmp-wrappers__1.o
collect2: ld returned 1 exit status
make[1]: *** [libraries/integer-gmp/dist-install/build/cbits/gmp-wrappers.p_o] Error 1