CLI equivalent for 'lib-dir' configuration file option

[kinke]

Hi! Yeah, there's no CLI equivalent for `lib-dirs`, which is 
special. See e.g. the `libDirs` occurrences in 
https://github.com/ldc-developers/ldc/blob/master/driver/linker-msvc.cpp. What you are missing in this specific case is linking `ldc_rt.builtins.lib`, which the compiler only does implicitly if found in `lib-dirs`, as for other libraries (not guaranteed to be bundled with the LDC installation) and object files.