Oh, the ```CPP_MANGLE``` macro is always defined for gdc, then that's easy.

I was wondering whether I should do this:
```d
if (findCondition(global.params.versionids, new Identifier("OSX", 0)))
  global.params.isOSX = true;
```
after the call to ```TARGET_OS_D_BUILTINS()``` or whether the ``` global.params.isOSX = true;``` code should be added directly to the ````TARGET_OS_D_BUILTINS``` macro in ```config/darwin.h```.