Missing D files

[LorenDB]

I'm trying to learn D, but at the same time I'm trying to 
generate a Qt binding for D (yes, yes, Qte5, but I don't like the 
dynamic loading of libraries that Qte5 uses). I know that this is 
probably a bad stage of the game to be making bindings, but...

Anyway, I'm using Swig to create a wrapper for the Qt files. 
While creating the first wrapper (QObject), I ran across some 
errors:

```
Performing "debug" build using ldc2 for x86_64.
qt5-d ~master: building configuration "application"...
source/QtCore/QObject_im.d(51,19): Error: module string is in 
file 'std/c/string.d' which cannot be read
import path[0] = source/
import path[1] = /usr/lib/ldc/x86_64-linux-gnu/include/d
import path[2] = /usr/include/d
source/QtCore/QObject_im.d(121,14): Error: module linux is in 
file 'std/c/linux/linux.d' which cannot be read
import path[0] = source/
import path[1] = /usr/lib/ldc/x86_64-linux-gnu/include/d
import path[2] = /usr/include/d
ldc2 failed with exit code 1.
```

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I have no idea where I'm supposed to get "std/c/linux/linux.d" 
and "std/c/string.d". I'm on Ubuntu 20.04 using the bundled ldc2 
and corresponding libphobos. Any suggestions?