Fix linkage errors on Windows in header-only config

format.cc

@@ -444,7 +444,7 @@ FMT_FUNC void fmt::internal::report_unknown_type(char code, const char *type) {
 
 #ifdef _WIN32
 
-fmt::internal::UTF8ToUTF16::UTF8ToUTF16(fmt::StringRef s) {
+FMT_FUNC fmt::internal::UTF8ToUTF16::UTF8ToUTF16(fmt::StringRef s) {
   int length = MultiByteToWideChar(
       CP_UTF8, MB_ERR_INVALID_CHARS, s.c_str(), -1, 0, 0);
   static const char ERROR[] = "cannot convert string from UTF-8 to UTF-16";
@@ -457,14 +457,14 @@ fmt::internal::UTF8ToUTF16::UTF8ToUTF16(fmt::StringRef s) {
     FMT_THROW(WindowsError(GetLastError(), ERROR));
 }
 
-fmt::internal::UTF16ToUTF8::UTF16ToUTF8(fmt::WStringRef s) {
+FMT_FUNC fmt::internal::UTF16ToUTF8::UTF16ToUTF8(fmt::WStringRef s) {
   if (int error_code = convert(s)) {
     FMT_THROW(WindowsError(error_code,
         "cannot convert string from UTF-16 to UTF-8"));
   }
 }
 
-int fmt::internal::UTF16ToUTF8::convert(fmt::WStringRef s) {
+FMT_FUNC int fmt::internal::UTF16ToUTF8::convert(fmt::WStringRef s) {
   int length = WideCharToMultiByte(CP_UTF8, 0, s.c_str(), -1, 0, 0, 0, 0);
   if (length == 0)
     return GetLastError();
@@ -476,7 +476,7 @@ int fmt::internal::UTF16ToUTF8::convert(fmt::WStringRef s) {
   return 0;
 }
 
-void fmt::WindowsError::init(
+FMT_FUNC void fmt::WindowsError::init(
     int error_code, StringRef format_str, ArgList args) {
   error_code_ = error_code;
   MemoryWriter w;
@@ -509,7 +509,7 @@ FMT_FUNC void fmt::internal::format_system_error(
 }
 
 #ifdef _WIN32
-void fmt::internal::format_windows_error(
+FMT_FUNC void fmt::internal::format_windows_error(
     fmt::Writer &out, int error_code,
     fmt::StringRef message) FMT_NOEXCEPT(true) {
   class String {
@@ -1069,7 +1069,7 @@ FMT_FUNC void fmt::report_system_error(
 }
 
 #ifdef _WIN32
-void fmt::report_windows_error(
+FMT_FUNC void fmt::report_windows_error(
     int error_code, fmt::StringRef message) FMT_NOEXCEPT(true) {
   report_error(internal::format_windows_error, error_code, message);
 }