Is there a way to do a C++ style compile-time assertion to determine machine’s endianness?

If you’re using autoconf, you can use the AC_C_BIGENDIAN macro, which is fairly guaranteed to work (setting the WORDS_BIGENDIAN define by default)

alternately, you could try something like the following (taken from autoconf) to get a test that will probably be optimized away (GCC, at least, removes the other branch)

int is_big_endian()
{
    union {
        long int l;
        char c[sizeof (long int)];
    } u;

    u.l = 1;

    if (u.c[sizeof(long int)-1] == 1)
    {
        return 1;
    }
    else
        return 0;
}