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)

It is not exposed as a CMake option but one can define it manually
in the fmt subproject, e.g.:

    target_compile_definitions(fmt PUBLIC FMT_WINDOWS_NO_WCHAR)

Fixes #3631

include/fmt/core.h

@@ -2675,7 +2675,7 @@ void vformat_to(buffer<Char>& buf, basic_string_view<Char> fmt,
                 typename vformat_args<Char>::type args, locale_ref loc = {});
 
 FMT_API void vprint_mojibake(std::FILE*, string_view, format_args);
-#ifndef _WIN32
+#if !defined(_WIN32) || defined(FMT_WINDOWS_NO_WCHAR)
 inline void vprint_mojibake(std::FILE*, string_view, format_args) {}
 #endif
 }  // namespace detail

include/fmt/format-inl.h

@@ -18,7 +18,7 @@
 #  include <locale>
 #endif
 
-#ifdef _WIN32
+#if defined(_WIN32) && !defined(FMT_WINDOWS_NO_WCHAR)
 #  include <io.h>  // _isatty
 #endif
 
@@ -1426,7 +1426,7 @@ 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>;

test/format-impl-test.cc

@@ -351,7 +351,7 @@ TEST(format_impl_test, write_dragon_even) {
   if (!FMT_MSC_VERSION) EXPECT_EQ(s, "33554450");
 }
 
-#ifdef _WIN32
+#if defined(_WIN32) && !defined(FMT_WINDOWS_NO_WCHAR)
 #  include <windows.h>
 
 TEST(format_impl_test, write_console_signature) {