diff --git a/README.md b/README.md
index fa30bd0..56be3e5 100644
--- a/README.md
+++ b/README.md
@@ -137,11 +137,13 @@ continuations. For example:
 
 ```
 // HAVE_BAR
+
+#undef HAVE_BAR
+
+/* No bar on Windows except with MinGW. */
 #if !defined(_WIN32) || \
      defined(__MINGW32__)
 #  define HAVE_BAR 1
-#else
-#  undef HAVE_BAR /* No bar on Windows except with MinGW. */
 #endif
 ```
 
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STDLIB_H.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STDLIB_H.h
index 6c091bd..495c174 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STDLIB_H.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STDLIB_H.h
@@ -1,2 +1,5 @@
 // HAVE_STDLIB_H
+
+#undef HAVE_STDLIB_H
+
 #define HAVE_STDLIB_H 1
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STRLCAT.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STRLCAT.h
index a626541..3bf1d9e 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STRLCAT.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STRLCAT.h
@@ -1,9 +1,10 @@
 // HAVE_STRLCAT
+
+#undef HAVE_STRLCAT
+
 #if defined(__FreeBSD__) || \
     defined(__OpenBSD__) || \
     defined(__NetBSD__)  || \
     defined(__APPLE__)
 #  define HAVE_STRLCAT 1
-#else
-#  undef HAVE_STRLCAT
 #endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STRLCPY.h b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STRLCPY.h
index f0ff465..0998aa5 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STRLCPY.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/HAVE_STRLCPY.h
@@ -1,9 +1,10 @@
 // HAVE_STRLCPY
+
+#undef HAVE_STRLCPY
+
 #if defined(__FreeBSD__) || \
     defined(__OpenBSD__) || \
     defined(__NetBSD__)  || \
     defined(__APPLE__)
 #  define HAVE_STRLCPY 1
-#else
-#  undef HAVE_STRLCPY
 #endif
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/STDC_HEADERS.h b/libbuild2-autoconf/libbuild2/autoconf/checks/STDC_HEADERS.h
index 687005c..26ff841 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/STDC_HEADERS.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/STDC_HEADERS.h
@@ -1,2 +1,5 @@
 // STDC_HEADERS
+
+#undef STDC_HEADERS
+
 #define STDC_HEADERS 1