diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h b/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h
index 35ab553d..5e64cf57 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/BYTE_ORDER.h
@@ -1,56 +1,70 @@
 // BYTE_ORDER!
 
-/* Include the endianness header based on platform.
+/* Detect BYTE_ORDER/BIG_ENDIAN/LITTLE_ENDIAN
+ * Prefer compiler built-ins first, then common headers, with a Windows
+ * little-endian fallback. Some platforms gate macro visibility behind
+ * feature macros like _ANSI_SOURCE, _POSIX_C_SOURCE, _XOPEN_SOURCE, or
+ * _NETBSD_SOURCE; in those cases headers may only expose underscored
+ * forms (e.g., __BYTE_ORDER or _BYTE_ORDER). This code aliases those to
+ * the non-underscored names when found.
  *
- * 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).
+ * Available on OpenBSD 2.0, FreeBSD 3.0, NetBSD 1.3, macOS 10.0,
+ * Linux/glibc 2.0, Solaris 10, and Windows including MinGW & Cygwin.
  */
-#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
+// endian detection — prefer compiler built-ins, then headers, then
+// Windows fallback.
+#ifndef BYTE_ORDER
+   // 1) Compiler built-ins (GCC/Clang)
+#  if defined(__BYTE_ORDER__)       && \
+      defined(__ORDER_BIG_ENDIAN__) && \
+      defined(__ORDER_LITTLE_ENDIAN__)
+#    define BYTE_ORDER     __BYTE_ORDER__
+#    define BIG_ENDIAN     __ORDER_BIG_ENDIAN__
+#    define LITTLE_ENDIAN  __ORDER_LITTLE_ENDIAN__
+   // 2) Try common headers if present
+#  elif defined(__has_include)
+#    if __has_include(<endian.h>)
+#      include <endian.h>
+#    elif __has_include(<sys/endian.h>)
+#      include <sys/endian.h>
+#    elif __has_include(<machine/endian.h>)
+       // NOTE: This header is deprecated on macOS and can pollute the
+       //       namespace (e.g., u_int). It should not be necessary in
+       //       practice; included only as a last resort.
+#      include <machine/endian.h>
+#    elif !defined(_WIN32) && __has_include(<sys/param.h>)
+#      include <sys/param.h>
 #    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
+
+   // 3) If headers gave us underscored names, alias them.
+#  if !defined(BYTE_ORDER)
+#    if defined(__BYTE_ORDER) && \
+        defined(__BIG_ENDIAN) && \
+        defined(__LITTLE_ENDIAN)
+#      define BYTE_ORDER     __BYTE_ORDER
+#      define BIG_ENDIAN     __BIG_ENDIAN
+#      define LITTLE_ENDIAN  __LITTLE_ENDIAN
+#    elif defined(_BYTE_ORDER) && \
+          defined(_BIG_ENDIAN) && \
+          defined(_LITTLE_ENDIAN)
+#      define BYTE_ORDER     _BYTE_ORDER
+#      define BIG_ENDIAN     _BIG_ENDIAN
+#      define LITTLE_ENDIAN  _LITTLE_ENDIAN
+#    elif defined(__DARWIN_BYTE_ORDER) && \
+          defined(__DARWIN_BIG_ENDIAN) && \
+          defined(__DARWIN_LITTLE_ENDIAN)
+#      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
+   // 4) Windows (MSVC/MinGW/ClangCL) — all support Windows ABIs & are LE.
+#  if !defined(BYTE_ORDER) && defined(_WIN32)
+#    define BIG_ENDIAN     4321
+#    define LITTLE_ENDIAN  1234
+#    define BYTE_ORDER     LITTLE_ENDIAN
 #  endif
 #endif