Takenobu Tani authored Apr 14, 2019 and Ben Gamari committed Jun 12, 2019

To cover ci conditions from ghc8.6 to 8.9, I add `-haddock` option
to `mk/flavours/perf.mk` rather than `.circleci/prepare-system.sh`.

Because in windows condition of ghc-8.9, `mk/flavours/*` is included
after `prepare-system.sh`.

In addition, in linux condition of ghc-8.6, `mk/flavors/perf.mk` is used.

(cherry picked from commit 43a39c3c)