[configure] reset config.prefix if not set by the user

configure.ac

@@ -89,6 +89,10 @@ fi
 ##########################################
 case $prefix in
   *\'*|*\"* ) AC_MSG_ERROR(quotes not allowed in --prefix argument: $prefix);;
+  NONE )
+    # no prefix: remove existing config.prefix if any
+    rm -f config.prefix
+    ;;
   * )
     echo "FRAMAC_INSTALLDIR?=\"$prefix\"" > config.prefix
     ;;