Fix broken scram-sha-256 authentication on big-endian machies.

Fix broken scram-sha-256 authentication on big-endian machies.

When scram-sha-256 authentication is performed, a hash function
pg_sha_256_final is used. It was imported from PostgreSQL and it uses
preprocessor define WORDS_BIGENDIAN to judge host machine's
endianness. Although WORDS_BIGENDIAN should be defined while
configure, this part was missed when pg_sha_256_final (and others) was
imported from PostgreSQL. As a result, scram-sha-256 worked only in
little endian machines. This commit fixes the issue by adding
AC_C_BIGENDIAN macro to configure.ac.

Author: Tatsuo Ishii
Reported-by: Christoph Berg
Reviewed-by: pranavkaruvally
Discussion: https://github.com/pgpool/pgpool2/issues/106
Backpatch-through: v4.2