diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_POINTER.h b/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_POINTER.h
index b832971..1f258fe 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_POINTER.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_POINTER.h
@@ -1,6 +1,6 @@
 // SIZEOF_POINTER
 
-/* See also SIZEOF_SIZE_T (the two may not be the same). */
+/* See also SIZEOF_SIZE_T (the two may not be the same), SIZEOF_VOID_P. */
 
 #undef SIZEOF_POINTER
 
diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_VOID_P.h b/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_VOID_P.h
index 61d3773..e6e98c0 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_VOID_P.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_VOID_P.h
@@ -1,11 +1,23 @@
 // SIZEOF_VOID_P
 
+/* See also SIZEOF_SIZE_T (the two may not be the same), SIZEOF_POINTER. */
+
 #undef SIZEOF_VOID_P
 
-/* @@ TODO: redo using SIZEOF_POINTER (see SIZEOF_SIZE_T). */
-
-#if defined(_WIN64) || defined(__x86_64__) || defined(__ppc64__)
-#  define SIZEOF_VOID_P 8
+#ifdef _MSC_VER
+/* _WIN64 is defined for both x64 (x86_64) and ARM64 (aarch64). */
+#  ifdef _WIN64
+#    define SIZEOF_VOID_P 8
+#  else
+#    define SIZEOF_VOID_P 4
+#  endif
 #else
-#  define SIZEOF_VOID_P 4
+/* Both GCC and Clang (and maybe others) define __SIZEOF_POINTER__. */
+#  ifdef __SIZEOF_POINTER__
+#    define SIZEOF_VOID_P __SIZEOF_POINTER__
+#  endif
+#endif
+
+#ifndef SIZEOF_VOID_P
+#  error unable to determine size of pointer
 #endif