Use a more robust detection of __builtin_bswap funtions for Clang. Fix Clang lack of __builtin_bswap16 in some releases. To be conservative only, fallback to byteswap.h if __linux__ is defined. Fix intrinsic_test so that <cstdint> isn't required.
Fix failure to define BOOST_ENDIAN_INTRINSIC_MSG when user defines BOOST_ENDIAN_NO_INTRINSICS