diff --git a/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_OFF_T.h b/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_OFF_T.h
index a7eac23..17f49ef 100644
--- a/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_OFF_T.h
+++ b/libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_OFF_T.h
@@ -2,13 +2,18 @@
 
 #undef SIZEOF_OFF_T
 
-/* Note that the _FILE_OFFSET_BITS macro is expected to be defined by the
+
+/* Use __SIZEOF_SIZE_T__ (defined by GCC and Clang and maybe others) to detect
+ * a 64-bit target.
+ *
+ * Note that the _FILE_OFFSET_BITS macro is expected to be defined by the
  * user, for example, on the command line.
  */
 
 #ifdef _MSC_VER
 #  define SIZEOF_OFF_T 4
-#elif defined(__x86_64__) || defined(__ppc64__) || (defined(_FILE_OFFSET_BITS) && _FILE_OFFSET_BITS == 64)
+#elif (defined(__SIZEOF_SIZE_T__) && __SIZEOF_SIZE_T__ == 8) || \
+      (defined(_FILE_OFFSET_BITS) && _FILE_OFFSET_BITS == 64)
 #  define SIZEOF_OFF_T 8
 #else
 #  define SIZEOF_OFF_T 4