Disable warning about format string (#2067)

Reported by MinGW/GCC 10

include/fmt/chrono.h

@@ -383,12 +383,21 @@ inline std::tm gmtime(
 namespace detail {
 inline size_t strftime(char* str, size_t count, const char* format,
                        const std::tm* time) {
-  return std::strftime(str, count, format, time);
+  // Assign to a pointer to suppress GCCs -Wformat-nonliteral
+  // First assign the nullptr to suppress -Wsuggest-attribute=format
+  std::size_t (*strftime)(char*, std::size_t, const char*, const std::tm*) =
+      nullptr;
+  strftime = std::strftime;
+  return strftime(str, count, format, time);
 }
 
 inline size_t strftime(wchar_t* str, size_t count, const wchar_t* format,
                        const std::tm* time) {
-  return std::wcsftime(str, count, format, time);
+  // See above
+  std::size_t (*wcsftime)(wchar_t*, std::size_t, const wchar_t*,
+                          const std::tm*) = nullptr;
+  wcsftime = std::wcsftime;
+  return wcsftime(str, count, format, time);
 }
 }  // namespace detail