voidwrap: add explicit "voidwrap" target to Makefile so I can "make voidwrap" and not "make voidwrap_steam_x64.dll", strip .dll after building, add UNREFERENCED_PARAMETER to DllMain() to silence warnings

It's nice that the Steam SDK was apparently updated to be compatible with MinGW at some point, so we aren't forced to build this .dll with MSVC anymore.