fmt: Adjust to support MSVC

- there is no __attribute__ on that compiler
- sadly va_start is rejected to work on reference:

    z:\home\kirr\src\tools\go\pygo-win\BuildTools\vc\tools\msvc\14.35.32215\include\vadefs.h(194): error C2338: static_assert failed: 'va_start argument must not have reference type and must not be parenthesized'
    golang/fmt.cpp(53): note: see reference to class template instantiation '__vcrt_assert_va_start_is_not_reference<const golang::string &>' being compiled

  -> change format from string& to be string in sprintf.

  Probably it should be ok in practice from performance point of view because
  string contains only pointer to data, not the data itself.

golang/fmt.cpp

-// Copyright (C) 2019-2020  Nexedi SA and Contributors.
+// Copyright (C) 2019-2023  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -48,7 +48,7 @@ string _vsprintf(const char *format, va_list argp) {
     return string(buf.get(), buf.get() + nchar); // without trailing '\0'
 }
 
-string sprintf(const string &format, ...) {
+string sprintf(const string format, ...) {
     va_list argp;
     va_start(argp, format);
     string str = fmt::_vsprintf(format.c_str(), argp);
@@ -64,7 +64,7 @@ string sprintf(const char *format, ...) {
     return str;
 }
 
-error ___errorf(const string &format, ...) {
+error ___errorf(const string format, ...) {
     va_list argp;
     va_start(argp, format);
     error err = errors::New(fmt::_vsprintf(format.c_str(), argp));

golang/fmt.h

 #ifndef _NXD_LIBGOLANG_FMT_H
 #define _NXD_LIBGOLANG_FMT_H
 
-// Copyright (C) 2019-2020  Nexedi SA and Contributors.
+// Copyright (C) 2019-2023  Nexedi SA and Contributors.
 //                          Kirill Smelkov <kirr@nexedi.com>
 //
 // This program is free software: you can Use, Study, Modify and Redistribute
@@ -40,7 +40,7 @@ namespace golang {
 namespace fmt {
 
 // sprintf formats text into string.
-LIBGOLANG_API string sprintf(const string &format, ...);
+LIBGOLANG_API string sprintf(const string format, ...);
 
 
 // intseq<i1, i2, ...> and intrange<n> are used by errorf to handle %w.
@@ -75,7 +75,7 @@ namespace {
 //
 // format suffix ": %w" is additionally handled as in Go with
 // `errorf("... : %w", ..., err)` creating error that can be unwrapped back to err.
-LIBGOLANG_API error ___errorf(const string& format, ...);
+LIBGOLANG_API error ___errorf(const string format, ...);
 LIBGOLANG_API error ___errorfTryWrap(const string& format, error last_err, ...);
 LIBGOLANG_API string ___error_str(error err);
 
@@ -111,7 +111,10 @@ inline error errorf(const string& format, Argv... argv) {
 // `const char *` overloads just to catch format mistakes as
 // __attribute__(format) does not work with std::string.
 LIBGOLANG_API string sprintf(const char *format, ...)
-                                __attribute__ ((format (printf, 1, 2)));
+#ifndef _MSC_VER
+                                __attribute__ ((format (printf, 1, 2)))
+#endif
+	;
 
 // cannot use __attribute__(format) for errorf as we add %w handling.
 // still `const char *` overload is useful for performance.