Add an option to avoid wchar APIs on Windows

With this, fmt can be used on Windows 98 and the Original Xbox with:

    set(FMT_OS OFF)

`FMT_WINDOWS_NO_WCHAR` is detected automatically in 2 cases:

1. Missing `<io.h>` on the original Xbox
2. A defined `WINVER` on properly configured Windows projects

In other cases, one can `add_definitions(FMT_WINDOWS_NO_WCHAR)`
in the fmt subproject.

Fixes fmtlib#3631

include/fmt/format-inl.h

@@ -18,9 +18,18 @@
 #  include <locale>
 #endif
 
+#include "core.h"
+
 #ifdef _WIN32
+#if !defined(FMT_WINDOWS_NO_WCHAR) && (!FMT_HAS_INCLUDE(<io.h>) || (defined(WINVER) && WINVER <= 0x0500 && !defined(_WIN32_WINNT)))
+// A legacy Windows platform without wide char APIs, e.g. Windows 98, original Xbox.
+#define FMT_WINDOWS_NO_WCHAR
+#endif
+
+#ifndef FMT_WINDOWS_NO_WCHAR
 #  include <io.h>  // _isatty
 #endif
+#endif
 
 #include "format.h"
 
@@ -1426,7 +1435,7 @@ FMT_FUNC std::string vformat(string_view fmt, format_args args) {
 }
 
 namespace detail {
-#ifndef _WIN32
+#if !defined(_WIN32) || defined(FMT_WINDOWS_NO_WCHAR)
 FMT_FUNC bool write_console(std::FILE*, string_view) { return false; }
 #else
 using dword = conditional_t<sizeof(long) == 4, unsigned long, unsigned>;