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)
    set(FMT_WINDOWS_NO_WCHAR ON)

Fixes fmtlib#3631

CMakeLists.txt

@@ -151,6 +151,7 @@ option(FMT_TEST "Generate the test target." ${FMT_MASTER_PROJECT})
 option(FMT_FUZZ "Generate the fuzz target." OFF)
 option(FMT_CUDA_TEST "Generate the cuda-test target." OFF)
 option(FMT_OS "Include core requiring OS (Windows/Posix) " ON)
+option(FMT_WINDOWS_NO_WCHAR "Do not use wide-char APIs, such as WriteConsoleW, on Windows" OFF)
 option(FMT_MODULE "Build a module instead of a traditional library." OFF)
 option(FMT_SYSTEM_HEADERS "Expose headers with marking them as system." OFF)
 
@@ -290,6 +291,10 @@ if (FMT_MODULE)
   enable_module(fmt)
 endif ()
 
+if (FMT_WINDOWS_NO_WCHAR)
+  target_compile_definitions(fmt PUBLIC FMT_WINDOWS_NO_WCHAR)
+endif ()
+
 if (FMT_WERROR)
   target_compile_options(fmt PRIVATE ${WERROR_FLAG})
 endif ()

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>;