endian_buffer is not a POD in C++03

[viboes]

Even when we define

#define BOOST_ENDIAN_FORCE_PODNESS
#define  BOOST_ENDIAN_NO_CTORS

endian_buffer is not a POD, because it uses a protected data member.

See https://wandbox.org/permlink/PSYwP0oCoJ9cNtkV

Defining it as public in C++03 solves the issue.

std::is_pod works in C++11 as the definition was relaxed.
The problem is that boost::is_pod is based on C++03 definition of POD.

Formal definition from the C++ standard (C++03 8.5.1 §1):

An aggregate is an array or a class (clause 9) with no user-declared constructors (12.1), no private or protected non-static data members (clause 11), no base classes (clause 10), and no virtual functions (10.3).

Formal definition from the C++ standard (C++03 9 §4):

A POD-struct is an aggregate class that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-defined copy assignment operator and no user-defined destructor. Similarly, a POD-union is an aggregate union that has no non-static data members of type non-POD-struct, non-POD-union (or array of such types) or reference, and has no user-defined copy assignment operator and no user-defined destructor. A POD class is a class that is either a POD-struct or a POD-union.

[pdimov]

It's true that endian_buffer is not a C++03 POD, but there's a test that it can be used in a union, and this test passes. So I'm not sure that making the data member public is really necessary in practice.

[pdimov]

Should be fixed in develop.