constexpr Endian conversion in c++14
In all, this looks pretty solid to me, so I have only a few suggestions:

Eliminate spurious inline

The inline keyword is only useful if this is moved to a header file, so it's not strictly needed in the context of the code in the question. 

Consider defining a string literal

Here's a small enhancement: provide the ability to define a string literal.  Here's how it might be done:

constexpr uint32_t operator"" _be32(unsigned long long num) {
    return htobe32(num);
}


Usage:

printf("%16x %16x\n",   0x11223344,     0x11223344_be32     );


This makes things very tidy and easy to use.  Naturally, 16- and 64-bit versions are almost identical except for return type and name.