This allows to workaround the following error when using clang >= 21 on the following code:
```
#include <string>
#include <iostream>
#include "boost/lexical_cast.hpp"
enum SomeEnum
{
OneValue
};
int main()
{
std::cout << boost::lexical_cast<std::string>(SomeEnum::OneValue) << std::endl;
}
```
Results in:
```
/app/boost/include/boost/type_traits/is_signed.hpp:37:25: error: in-class initializer for static data member is not a constant expression
37 | static const no_cv_t minus_one = (static_cast<no_cv_t>(-1));
... (snapped)
/app/boost/include/boost/type_traits/is_signed.hpp:37:38: note: integer value -1 is outside the valid range of values [0, 1] for the enumeration type 'SomeEnum'
37 | static const no_cv_t minus_one = (static_cast<no_cv_t>(-1));
```
* C++ Standard Library based streams now constructed in less cases leading to **better performance**;
* less template instantiations and simpler maintainance;
* support for `volatile` input types was dropped, following the C++ Standard Library trend.
