Receiving "pthread_mutexattr_init" during compilation when using FileLogger.

[tastyminerals]

I getting
```
import/core/sync/mutex.d(87,36): Error: `pthread_mutexattr_init` 
cannot be interpreted at compile time, because it has no 
available source code
```
compile time error. One of those D exceptions which doesn't say 
where it happened in your code so you need to comment out the 
lines that might cause it one by one :D

I found that this was caused by the "FileLogger" which I create 
at the top of the file. But why?

```d
import std.experimental.logger: FileLogger;

auto fileLogger = new FileLogger("last.log");
```

Moving the FileLogger instantiation to a function fixes the issue 
though.