Merge #787: Use preprocessor macros instead of autoconf to detect endianness

Merge #787: Use preprocessor macros instead of autoconf to detect endianness

0dccf98a21beb245f6cd9ed76fb7368529df09c7 Use preprocessor macros instead of autoconf to detect endianness (Tim Ruffing)

Pull request description:

  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.

  Supersedes #770 .

ACKs for top commit:
  sipa:
    ACK 0dccf98a21beb245f6cd9ed76fb7368529df09c7
  gmaxwell:
    ACK 0dccf98a21beb245f6cd9ed76fb7368529df09c7

Tree-SHA512: 6779458de5cb6eaef2ac37f9d4b8fa6c9b299f58f6e5b72f2b0d7e36c12ea06074e483acfb85085a147e0f4b51cd67d897f61a67250ec1cea284a0f7680eb2e8