Fix SIZEOF_OFF_T check for MinGW

libbuild2-autoconf-tests/checks/sizeof/driver.c

@@ -20,4 +20,6 @@ main ()
 #ifdef TEST_PTHREAD
   assert (SIZEOF_PTHREAD_T == sizeof (pthread_t));
 #endif
+
+  return 0;
 }

libbuild2-autoconf/libbuild2/autoconf/checks/SIZEOF_OFF_T.h

@@ -8,10 +8,19 @@
  *
  * Note that the _FILE_OFFSET_BITS macro is expected to be defined by the
  * user, for example, on the command line.
+ *
+ * MinGW uses 32-bit off_t both in the 32-bit and 64-bit modes unless forced
+ * with _FILE_OFFSET_BITS.
  */
 
 #ifdef _MSC_VER
 #  define SIZEOF_OFF_T 4
+#elif defined(__MINGW32__)
+#  if defined(_FILE_OFFSET_BITS) && _FILE_OFFSET_BITS == 64
+#    define SIZEOF_OFF_T 8
+#  else
+#    define SIZEOF_OFF_T 4
+#  endif
 #elif (defined(__SIZEOF_SIZE_T__) && __SIZEOF_SIZE_T__ == 8) || \
       (defined(_FILE_OFFSET_BITS) && _FILE_OFFSET_BITS == 64)
 #  define SIZEOF_OFF_T 8