diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/BUILD2_AUTOCONF_LIBC_VERSION.h b/libbuild2-autoconf/libbuild2/autoconf/checks/BUILD2_AUTOCONF_LIBC_VERSION.h
index 7f5705f..dad33e0 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/BUILD2_AUTOCONF_LIBC_VERSION.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/BUILD2_AUTOCONF_LIBC_VERSION.h
@@ -31,9 +31,11 @@
  * #if defined(__FreeBSD__)
  * #if defined(__OpenBSD__)
  * #if defined(__NetBSD__)
- * #if defined(__APPLE__) && defined(__MACH__)
  *
- * Note that just __APPLE__ covers all the Apple platforms (MacOS, iOS, etc).
+ * Except for MacOS, which we detect using our own macro (for the sake of
+ * simplicity):
+ *
+ * #if defined(BUILD2_AUTOCONF_MACOS)
  *
  * Macros for detecting platforms and their versions:
  *
@@ -155,8 +157,16 @@
  *
  * Return 1 if the given version number is >= the Mac OS version, or 0
  * otherwise.
+ *
+ * BUILD2_AUTOCONF_MACOS
+ *
+ * Defined if the current platform is MacOS, or undefined otherwise.
  */
 #if defined(__APPLE__) && defined(__MACH__)
+/* Note that just __APPLE__ covers all the Apple platforms (MacOS, iOS, etc).
+ */
+#  define BUILD2_AUTOCONF_MACOS 1
+
 #  if !defined(__MAC_OS_X_VERSION_MIN_REQUIRED)
 #    error __MAC_OS_X_VERSION_MIN_REQUIRED not defined
 #  endif
@@ -168,6 +178,8 @@
        (__MAC_OS_X_VERSION_MIN_REQUIRED >= (maj)*10000 + (min)*100)
 #  endif
 #else
+#  undef BUILD2_AUTOCONF_MACOS
+
 #  define BUILD2_AUTOCONF_MACOS_PREREQ(maj, min) 0
 #endif
 
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_DLOPEN.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_DLOPEN.h
index d30ad23..7672bae 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_DLOPEN.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_DLOPEN.h
@@ -12,6 +12,6 @@
     defined(__FreeBSD__)               || \
     defined(__OpenBSD__)               || \
     defined(__NetBSD__)                || \
-    (defined(__APPLE__) && defined(__MACH__))
+    defined(BUILD2_AUTOCONF_MACOS)
 #  define HAVE_DLOPEN 1
 #endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FCNTL_H.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FCNTL_H.h
index bb94506..2b4e75b 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FCNTL_H.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FCNTL_H.h
@@ -1,4 +1,8 @@
-// HAVE_FCNTL_H
+// HAVE_FCNTL_H : BUILD2_AUTOCONF_LIBC_VERSION
+
+#ifndef BUILD2_AUTOCONF_LIBC_VERSION
+#  error BUILD2_AUTOCONF_LIBC_VERSION appears to be conditionally included
+#endif
 
 #undef HAVE_FCNTL_H
 
@@ -6,6 +10,6 @@
     defined(__FreeBSD__) || \
     defined(__OpenBSD__) || \
     defined(__NetBSD__)  || \
-    (defined(__APPLE__) && defined(__MACH__))
+    defined(BUILD2_AUTOCONF_MACOS)
 #  define HAVE_FCNTL_H 1
 #endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMES.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMES.h
index b5b4579..4b8e9cf 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMES.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_FUTIMES.h
@@ -14,6 +14,6 @@
     BUILD2_AUTOCONF_FREEBSD_PREREQ(3, 0)   || \
     BUILD2_AUTOCONF_OPENBSD_PREREQ(199610) || \
     BUILD2_AUTOCONF_NETBSD_PREREQ(1, 2)    || \
-    (defined(__APPLE__) && defined(__MACH__))
+    defined(BUILD2_AUTOCONF_MACOS)
 #  define HAVE_FUTIMES 1
 #endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_LINKAT.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_LINKAT.h
index 47e3136..ca8191d 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_LINKAT.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_LINKAT.h
@@ -14,6 +14,6 @@
     BUILD2_AUTOCONF_FREEBSD_PREREQ(8, 0)   || \
     BUILD2_AUTOCONF_OPENBSD_PREREQ(201111) || \
     BUILD2_AUTOCONF_NETBSD_PREREQ(7, 0)    || \
-    (defined(__APPLE__) && defined(__MACH__))
+    defined(BUILD2_AUTOCONF_MACOS)
 #  define HAVE_LINKAT 1
 #endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLL.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLL.h
index 0426e53..4ec16d1 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLL.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_POLL.h
@@ -14,6 +14,6 @@
     defined(__FreeBSD__)                || \
     defined(__OpenBSD__)                || \
     BUILD2_AUTOCONF_NETBSD_PREREQ(1, 3) || \
-    (defined(__APPLE__) && defined(__MACH__))
+    defined(BUILD2_AUTOCONF_MACOS)
 #  define HAVE_POLL 1
 #endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_UNISTD_H.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_UNISTD_H.h
index 7d57a04..8c5ed52 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_UNISTD_H.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_UNISTD_H.h
@@ -1,4 +1,8 @@
-// HAVE_UNISTD_H
+// HAVE_UNISTD_H : BUILD2_AUTOCONF_LIBC_VERSION
+
+#ifndef BUILD2_AUTOCONF_LIBC_VERSION
+#  error BUILD2_AUTOCONF_LIBC_VERSION appears to be conditionally included
+#endif
 
 #undef HAVE_UNISTD_H
 
@@ -6,6 +10,6 @@
     defined(__FreeBSD__) || \
     defined(__OpenBSD__) || \
     defined(__NetBSD__)  || \
-    (defined(__APPLE__) && defined(__MACH__))
+    defined(BUILD2_AUTOCONF_MACOS)
 #  define HAVE_UNISTD_H 1
 #endif