12
The Windows APIs has always supported forward slash as directory separator. And that is because as far back as at least DOS 3.1 it was supported as well. The problem has always been COMMAND.COM and CMD.EXE. They use forward slash as the option indicator (instead of dash as found in Unix). Never ever use backslashes for directory separators in languages where a backslash in a string is used to escape special characters.
If you lack a Unix shell, don't you have an editor that does search/replace across multiple files? Heck, write a small program to do it if you have to. Parsing C++ source code for its #include statements can't be hard.