This code line is because some (old) versions of MinGW does not support wide chars in file opening interface.
If that has changed, this line should be changed too (with compiler/runtime/whatever version check).
We're open to contributions, so if you could investigate the problem in depth (what versions/runtimes works with wchar_t and what is not), just propose patch.

Alternatively, you may create own Libraw datastream implementation and pass it to LibRaw.