The 'ghc-flag' directive is already registered to domain std

[sjmackenzie]

Steps to reproduce

• nix-shell -p stack
• stack build

Expected behaviour

• it builds

Actual bedaviour

Extension error:
The 'ghc-flag' directive is already registered to domain std
make[1]: *** [docs/users_guide/ghc.mk:28: docs/users_guide/build-man/ghc.1] Error 2
make: *** [Makefile:127: all] Error 2
builder for '/nix/store/hc37mvicvrpaxibnwis4b9ah218wazqb-ghc-8.4.4.drv' failed with exit code 2
error: build of '/nix/store/hc37mvicvrpaxibnwis4b9ah218wazqb-ghc-8.4.4.drv' failed