compare.c (uuid_compare), copy.c (uuid_copy),

compare.c (uuid_compare), copy.c (uuid_copy),
	isnull.c (uuid_is_null), pack.c (uuid_pack),
	parse.c (uuid_parse), unpack.c (uuid_unpack),
	unparse.c (uuid_unparse), uuid.h, uuidP.h,
	uuid_time.c (uuid_time, uuid_type, uuid_variant):
	Use const for pointer variables that we don't modify.  Add
	the appropriate ifdef's in uuid.h to make it be C++ friendly.