2to3 detection for Windows Python distributions

The 2to3 standalone tool does not exist on Windows. Use the 2to3.py
script instead.

configure.ac

@@ -911,6 +911,17 @@ if test -n "$PYTHON3"; then
   if test "x$PY2TO3BIN" = xyes; then
     py3to2=`echo $PYTHON3 | sed -e "s/python/2to3-/"`
     AC_CHECK_PROGS(PY2TO3, $py3to2 2to3)
+    if test -z "$PY2TO3"; then
+      # Windows distributions don't always have the 2to3 executable
+      AC_MSG_CHECKING(for 2to3.py)
+      py2to3script="$PY3PREFIX/Tools/scripts/2to3.py"
+      if test -f "$py2to3script"; then
+        AC_MSG_RESULT($py2to3script)
+        PY2TO3="$PYTHON3 $py2to3script"
+      else
+        AC_MSG_RESULT(Not found)
+      fi
+    fi
   else
     PY2TO3="$PY2TO3BIN"
   fi