Support system installation

As discussed in #209, this is a pull request for allowing system installation of the Makefile in a way that will make it work without extra actions from the user.

The project Makefile must look like:

    BOARD_TAG=uno
    include $(shell arduino-mk-tool)

For people who don't use this installation procedure, nothing changes; everything continues to work the way it did.

For using this installation:

    autoreconf -fis
    ./configure
    make install

("make" is skipped because there is nothing to make.)

To get rid of all the junk that the autotools create:

    make autoclean

I did not add the ARDUINO_DIR for Windows, because I don't know where it is supposed to be, and if I read Arduino.mk, it seems to not be in a standard location.  This means it will need to be set manually when this installation is run on Windows.