pgcrypto BYTE_ORDER fix for stable branches

Recently was uncovered that pgcrypto does not include
right header file to get BYTE_ORDER define on Cygwin
and MINGW, and the missing define does not result in
build failures, but random combinations of little and
big-endian code sections.

This patch adds missing sys/param.h include for md5.c,
sha1.c and rijndael.c plus also check to catch undefined
BYTE_ORDER.

The patch applies cleanly and is tested on branches
for versions 7.2, 7.3, 7.4 and 8.0. There are also
no failures on buildfarm for HEAD, where this patch
is already included.

--
marko