Utility: comment includes similarly to the rest of Directory.cpp.
Doc++
Provide also CORRADE_TARGET_{UNIX,WINDOWS}.

I'm tired of searching for proper platform-specific definitions all
the time. Also it seems that none of `__unix`, `unix` or `__unix__` is
defined on Apple's Clang. AARGH.
Use CORRADE_TARGET_{UNIX,WINDOWS} throughout the code.

Should fix issues with Directory (and subsequently also with
PluginManager) on OSX.