/codepage:id
where:
If you compile one or more source code files that were not created to use the default code page on your computer, you can use the /codepage option to specify which code page should be used. /codepage applies to all source code files in your compilation.
If the source code files were created with the same codepage that is in effect on your computer or if the source code files were created with UNICODE or UTF-8, you need not use /codepage.
See GetCPInfo for information on how to find which code pages are supported on your system.