Harden byte order detection

[kkaefer]

Use __BYTE_ORDER__ for GCC/Clang, and include Linux/BSD-specific headers for endian detection, and a constexpr static_assert instead of silently defaulting to little-endian.

Fixes #135.

[kkaefer]

Clang and GCC define __BYTE_ORDER__ by default: https://gcc.gnu.org/onlinedocs/gcc-4.9.0/cpp/Common-Predefined-Macros.html

[joto]

If the is_little_endian() function can perfectly detect the endianness at compile time, why don't we get rid of all the macro shenenigans alltogether? The only thing this is used for is to call (or not) the byteswap_inplace() function. If we add an if (!is_little_endian()) condition inside that function, we should be okay. We can use if consexpr if we switch to c++17 (which I don't want to do because libosmium is still c++14, but it's not out of the question to do that step). But even with a simple if the compiler should be able to remove all that code in the little-endian case because it detects that is superfluous.

[kkaefer]

If the is_little_endian() function can perfectly detect the endianness at compile time, why don't we get rid of all the macro shenenigans alltogether?

We can't. It didn't work, and I removed it again after running actual big endian machines where this failed.

[joto]

We can't. It didn't work, ...

Okay, then I am fine with it once the CI problems are fixed.