Use preprocessor macros instead of autoconf to detect endianness

Use preprocessor macros instead of autoconf to detect endianness

This does not fix any particular issue but it's preferable to not
rely on autoconf. This avoids endianness mess for users on BE hosts
if they use their build without autoconf.

The macros are carefully written to err on the side of the caution,
e.g., we #error if the user manually configures a different endianness
than what we detect.