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diff --git a/build-aux/zw_endianness.m4 b/build-aux/zw_endianness.m4
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+dnl Written by Zack Weinberg <zackw at panix.com> in 2018.
+dnl To the extent possible under law, Zack Weinberg has waived all
+dnl copyright and related or neighboring rights to this work.
+dnl
+dnl See https://creativecommons.org/publicdomain/zero/1.0/ for further
+dnl details.
+dnl
+dnl This macro is like AC_C_BIGENDIAN, but it doesn't try to determine
+dnl a final value at configure time.  Instead, it probes for a set of
+dnl headers and compile-time macros that may or may not be available,
+dnl and uses them to guide preprocessor logic that makes the final
+dnl determination.  This works better with MacOS "universal binaries",
+dnl which may involve compiling the program twice under opposite
+dnl endiannesses; no fixed byte-order macro is correct in that case,
+dnl but the compiler's built-ins will be.  This approach is also
+dnl friendlier to cross-compilation.
+dnl
+dnl This is the preprocessor logic that should be put in an appropriate
+dnl location, after including config.h:
+dnl
+dnl #include <limits.h>
+dnl #ifdef HAVE_ENDIAN_H
+dnl #include <endian.h>
+dnl #endif
+dnl #ifdef HAVE_SYS_ENDIAN_H
+dnl #include <sys/endian.h>
+dnl #endif
+dnl #ifdef HAVE_SYS_PARAM_H
+dnl #include <sys/param.h>
+dnl #endif
+dnl
+dnl #if ENDIANNESS_IS_BIG
+dnl # define ENDIAN_BIG
+dnl #elif ENDIANNESS_IS_LITTLE
+dnl # define ENDIAN_LITTLE
+dnl #elif ENDIANNESS_IS_PDP
+dnl # define ENDIAN_PDP
+dnl #else
+dnl # error "Unable to determine byte order"
+dnl #endif
+dnl ------------------------------------------------
+
+dnl There's no good way to implement this macro as a _shell_ loop, but we
+dnl can reasonably implement it as an _m4_ loop that expands to a sequence
+dnl of conditionals.  Actually two sequences of conditionals, one inside
+dnl AC_CACHE_CHECK and one outside.
+
+m4_define([zw_C_ENDIANNESS_options], [
+[ [BYTE_ORDER and xxx_ENDIAN],
+  [defined BYTE_ORDER && defined BIG_ENDIAN && defined LITTLE_ENDIAN && BIG_ENDIAN != LITTLE_ENDIAN],
+  [BYTE_ORDER == BIG_ENDIAN],
+  [BYTE_ORDER == LITTLE_ENDIAN],
+  [BYTE_ORDER == PDP_ENDIAN],
+],
+[ [__BYTE_ORDER and __xxx_ENDIAN],
+  [defined __BYTE_ORDER && defined __BIG_ENDIAN && defined __LITTLE_ENDIAN && __BIG_ENDIAN != __LITTLE_ENDIAN],
+  [__BYTE_ORDER == __BIG_ENDIAN],
+  [__BYTE_ORDER == __LITTLE_ENDIAN],
+  [__BYTE_ORDER == __PDP_ENDIAN],
+],
+[ [__BYTE_ORDER__ and __xxx_ENDIAN__],
+  [defined __BYTE_ORDER__ && defined __BIG_ENDIAN__ && defined __LITTLE_ENDIAN__ && __BIG_ENDIAN__ != __LITTLE_ENDIAN__],
+  [__BYTE_ORDER__ == __BIG_ENDIAN__],
+  [__BYTE_ORDER__ == __LITTLE_ENDIAN__],
+  [__BYTE_ORDER__ == __PDP_ENDIAN__],
+],
+[ [__BYTE_ORDER__ and __ORDER_xxx_ENDIAN__],
+  [defined __BYTE_ORDER__ && defined __ORDER_BIG_ENDIAN__ && defined __ORDER_LITTLE_ENDIAN__ && __ORDER_BIG_ENDIAN__ != __ORDER_LITTLE_ENDIAN__],
+  [__BYTE_ORDER__ == __ORDER_BIG_ENDIAN__],
+  [__BYTE_ORDER__ == __ORDER_LITTLE_ENDIAN__],
+  [__BYTE_ORDER__ == __ORDER_PDP_ENDIAN__],
+],
+[ [_BIG_ENDIAN and _LITTLE_ENDIAN],
+  [(defined _BIG_ENDIAN) != (defined _LITTLE_ENDIAN)],
+  [defined _BIG_ENDIAN], [defined _LITTLE_ENDIAN], [0],
+],
+[ [__BIG_ENDIAN__ and __LITTLE_ENDIAN__],
+  [(defined __BIG_ENDIAN__) != (defined __LITTLE_ENDIAN__)],
+  [defined __BIG_ENDIAN__], [defined __LITTLE_ENDIAN__], [0],
+],
+[ [__ARMEB__ and __ARMEL__],
+  [(defined __ARMEB__) != (defined __ARMEL__)],
+  [defined __ARMEB__], [defined __ARMEL__], [0],
+],
+[ [__THUMBEB__ and __THUMBEL__],
+  [(defined __THUMBEB__) != (defined __THUMBEL__)],
+  [defined __THUMBEB__], [defined __THUMBEL__], [0],
+],
+[ [__AARCH64EB__ and __AARCH64EL__],
+  [(defined __AARCH64EB__) != (defined __AARCH64EL__)],
+  [defined __AARCH64EB__], [defined __AARCH64EL__], [0],
+],
+[ [__MIPSEB__ and __MIPSEL__],
+  [(defined __MIPSEB__) != (defined __MIPSEL__)],
+  [defined __MIPSEB__], [defined __MIPSEL__], [0],
+],
+[ [__MIPSEB and __MIPSEL],
+  [(defined __MIPSEB) != (defined __MIPSEL)],
+  [defined __MIPSEB], [defined __MIPSEL], [0],
+],
+[ [_MIPSEB and _MIPSEL],
+  [(defined _MIPSEB) != (defined _MIPSEL)],
+  [defined _MIPSEB], [defined _MIPSEL], [0],
+]
+])
+
+m4_define([zw_C_ENDIANNESS_test_one], [
+if test "$[]ac_cv_c_byte_order_macros" = unknown; then
+  AC_PREPROC_IFELSE([AC_LANG_PROGRAM([[
+#include <limits.h>
+#ifdef HAVE_ENDIAN_H
+#include <endian.h>
+#endif
+#ifdef HAVE_SYS_ENDIAN_H
+#include <sys/endian.h>
+#endif
+#ifdef HAVE_SYS_PARAM_H
+#include <sys/param.h>
+#endif
+#if !($2)
+#error fail
+#endif
+]])],
+  [ac_cv_c_byte_order_macros="$1"])
+fi])
+
+m4_define([zw_C_ENDIANNESS_case_one],
+  [["$1"], [
+  zw_C_ENDIANNESS_IS_BIG="($3)"
+  zw_C_ENDIANNESS_IS_LIT="($4)"
+  zw_C_ENDIANNESS_IS_PDP="($5)"]])
+
+AC_DEFUN([zw_C_ENDIANNESS], [
+AC_REQUIRE([AC_PROG_CC])
+AC_REQUIRE([AC_USE_SYSTEM_EXTENSIONS])
+AC_CHECK_HEADERS_ONCE([endian.h sys/endian.h sys/param.h])
+AC_CACHE_CHECK([for byte order macros],
+  [ac_cv_c_byte_order_macros], [
+  ac_cv_c_byte_order_macros=unknown
+  m4_map([zw_C_ENDIANNESS_test_one], [zw_C_ENDIANNESS_options])])
+
+AS_CASE([$ac_cv_c_byte_order_macros],
+        m4_map_sep([zw_C_ENDIANNESS_case_one], [,], [zw_C_ENDIANNESS_options]),
+        [AC_MSG_ERROR([unable to determine byte order at compile time])])
+
+AC_DEFINE_UNQUOTED([ENDIANNESS_IS_BIG], [$zw_C_ENDIANNESS_IS_BIG],
+  [Define as an @%:@if expression that is true when compiling for a big-endian CPU.])
+AC_DEFINE_UNQUOTED([ENDIANNESS_IS_LITTLE], [$zw_C_ENDIANNESS_IS_LIT],
+  [Define as an @%:@if expression that is true when compiling for a little-endian CPU.])
+AC_DEFINE_UNQUOTED([ENDIANNESS_IS_PDP], [$zw_C_ENDIANNESS_IS_PDP],
+  [Define as an @%:@if expression that is true when compiling for a PDP-endian CPU.])
+])