CircleCI always fails

[Quirin Schroll]

My PRs always fail on CircleCI due to this:
```
/home/circleci/dmd/compiler/test/../../druntime/import/core/stdc/config.d(325): Error: template instance `core.stdc.config._Complex!float` cannot resolve forward reference
     ref _Complex opAssign(_Complex!float  c) { re = c.re; im = 
c.im; return this; }
```

Is there a way to avoid this? CircleCI seems to work fine with 
every other PR.