[build.ps1] ensure the compiler is using UTF-8

[stevapple]

I am (again) proposing to add this flag because anyone who's not using 65001 code page could meet unexpected situations without it. Although it should have no effect with code page 65001 or clang-cl, I added a conditional check to eliminate the flag when unncessary.

[compnerd]

Do we have a guarantee that the code is stored in UTF-8 encoding? If we can get that put into .gitattributes as the guaranteed encoding, I don't think that this should be too big of a deal. Without the attributes, this could mis-parse the code.

[stevapple]

AFAIK all the current dependencies were encoded in UTF-8. Given the nature that Git only supports ASCII’s supersets without explicit working-tree-encoding in .gitattributes, I don’t think there’s any reason to do that for C/C++ source files.

BTW clang-cl.exe is implicitly /utf-8 and doesn’t care about code pages. If a source library supports clang-cl, it should not be in other encodings. Allowing other encoding could cause unnecessary confusion.