diff --git a/libbuild2-autoconf-tests/checks/byte-order/.gitignore b/libbuild2-autoconf-tests/checks/byte-order/.gitignore
new file mode 100644
index 0000000..534ba0c
--- /dev/null
+++ b/libbuild2-autoconf-tests/checks/byte-order/.gitignore
@@ -0,0 +1,2 @@
+driver
+driver-posix
diff --git a/libbuild2-autoconf-tests/checks/byte-order/buildfile b/libbuild2-autoconf-tests/checks/byte-order/buildfile
new file mode 100644
index 0000000..1d0346f
--- /dev/null
+++ b/libbuild2-autoconf-tests/checks/byte-order/buildfile
@@ -0,0 +1,23 @@
+# Build two executables: one with no macros defined and one with
+# _POSIX_C_SOURCE defined.
+#
+# _POSIX_C_SOURCE and similar macros can affect whether or not BYTE_ORDER,
+# BIG_ENDIAN, and LITTLE_ENDIAN are defined. Only do _POSIX_C_SOURCE here
+# because the others are used much less frequently.
+#
+
+./: exe{driver}: c{driver}
+
+./: h{config}: in{config}
+
+c.poptions += "-I$out_base"
+
+if ($c.target.system != 'win32-msvc')
+{
+  ./: exe{driver-posix}: obje{driver-posix}
+
+  obje{driver-posix}: c{driver}
+  {
+    c.poptions += -D_POSIX_C_SOURCE
+  }
+}
diff --git a/libbuild2-autoconf-tests/checks/byte-order/config.h.in b/libbuild2-autoconf-tests/checks/byte-order/config.h.in
new file mode 100644
index 0000000..d7b4313
--- /dev/null
+++ b/libbuild2-autoconf-tests/checks/byte-order/config.h.in
@@ -0,0 +1 @@
+#undef BYTE_ORDER
diff --git a/libbuild2-autoconf-tests/checks/byte-order/driver.c b/libbuild2-autoconf-tests/checks/byte-order/driver.c
new file mode 100644
index 0000000..9eea6d5
--- /dev/null
+++ b/libbuild2-autoconf-tests/checks/byte-order/driver.c
@@ -0,0 +1,39 @@
+#include <stdint.h> // uint32_t
+
+#include "config.h"
+
+/* Test that the byte order detected at runtime matches the one detected at
+ * compile time.
+ */
+
+/* Return:
+ *
+ *   0 if the runtime and compile time byte orders match
+ *
+ *   1 if the runtime and compile time byte orders do not match
+ *
+ *   2 if the BYTE_ORDER macro has a value other than BIG_ENDIAN or
+ *     LITTLE_ENDIAN
+ *
+ *   3 if not all of the byte order macros are defined
+ */
+int
+main ()
+{
+#if !defined(BYTE_ORDER) || !defined(BIG_ENDIAN) || !defined(LITTLE_ENDIAN)
+  return 3;
+#endif
+
+  union {
+    uint32_t i;
+    unsigned char a[4];
+  } u = {1};
+
+#if BYTE_ORDER == LITTLE_ENDIAN
+  return u.a[0] == 1 ? 0 : 1;
+#elif BYTE_ORDER == BIG_ENDIAN
+  return u.a[3] == 1 ? 0 : 1;
+#else
+  return 2;
+#endif
+}
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h b/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h
new file mode 100644
index 0000000..475dc32
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h
@@ -0,0 +1,59 @@
+// BYTE_ORDER!
+
+/* Include the endianness header based on platform.
+ *
+ * Each of these headers should define BYTE_ORDER, LITTLE_ENDIAN, BIG_ENDIAN,
+ * AND PDP_ENDIAN but this can be affected by macros like _ANSI_SOURCE,
+ * _POSIX_C_SOURCE, _XOPEN_SOURCE and _NETBSD_SOURCE, depending on the
+ * platform (in which case most of them define underscored versions only).
+ */
+#if defined(__GLIBC__) || defined(__OpenBSD__)
+#  include <endian.h>
+#elif defined(__FreeBSD__) || defined(__NetBSD__)
+#  include <sys/endian.h>
+#elif defined(__APPLE__)
+#  include <machine/endian.h>
+#elif !defined(_WIN32)
+#  include <sys/param.h>
+#endif
+
+/* Try various system- and compiler-specific byte order macro names if the
+ * endianness headers did not define BYTE_ORDER.
+ */
+#if !defined(BYTE_ORDER)
+#  if defined(__linux__)
+#    if defined(__BYTE_ORDER)
+#      define BYTE_ORDER    __BYTE_ORDER
+#      define BIG_ENDIAN    __BIG_ENDIAN
+#      define LITTLE_ENDIAN __LITTLE_ENDIAN
+#    endif
+#  elif defined(__FreeBSD__) || defined(__OpenBSD__) || defined(__NetBSD__)
+#    if defined(_BYTE_ORDER)
+#      define BYTE_ORDER    _BYTE_ORDER
+#      define BIG_ENDIAN    _BIG_ENDIAN
+#      define LITTLE_ENDIAN _LITTLE_ENDIAN
+#    endif
+#  elif defined(__APPLE__)
+#    if defined(__DARWIN_BYTE_ORDER)
+#      define BYTE_ORDER    __DARWIN_BYTE_ORDER
+#      define BIG_ENDIAN    __DARWIN_BIG_ENDIAN
+#      define LITTLE_ENDIAN __DARWIN_LITTLE_ENDIAN
+#    endif
+#  elif defined(_WIN32)
+#    define BIG_ENDIAN    4321
+#    define LITTLE_ENDIAN 1234
+#    define BYTE_ORDER    LITTLE_ENDIAN
+#  elif defined(__BYTE_ORDER__) &&              \
+        defined(__ORDER_BIG_ENDIAN__) &&        \
+        defined(__ORDER_LITTLE_ENDIAN__)
+     /* GCC, Clang (and others, potentially).
+      */
+#    define BYTE_ORDER    __BYTE_ORDER__
+#    define BIG_ENDIAN    __ORDER_BIG_ENDIAN__
+#    define LITTLE_ENDIAN __ORDER_LITTLE_ENDIAN__
+#  endif
+#endif
+
+#ifndef BYTE_ORDER
+#  error no byte order macros defined
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_BIGENDIAN.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_BIGENDIAN.h
new file mode 100644
index 0000000..87c27b0
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_BIGENDIAN.h
@@ -0,0 +1,7 @@
+// HAVE_BIGENDIAN : BYTE_ORDER
+
+#undef HAVE_BIGENDIAN
+
+#if BYTE_ORDER == BIG_ENDIAN
+#  define HAVE_BIGENDIAN 1
+#endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/WORDS_BIGENDIAN.h b/libbuild2-autoconf/libbuild2/autoconf/checks/WORDS_BIGENDIAN.h
new file mode 100644
index 0000000..caa21ae
--- /dev/null
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/WORDS_BIGENDIAN.h
@@ -0,0 +1,7 @@
+// WORDS_BIGENDIAN : BYTE_ORDER
+
+#undef WORDS_BIGENDIAN
+
+#if BYTE_ORDER == BIG_ENDIAN
+#  define WORDS_BIGENDIAN 1
+#endif