The Smoothie project needs to compile C++ inside config/boardname/src/ to use...

The Smoothie project needs to compile C++ inside config/boardname/src/ to use with High Priority Interruption, then I modified the board configs Makefile to support it, see attached patch.

It works fine for the first time compilation, but if we execute:

$ touch config/boardname/src/Pin.cxx

And execute "make" it will not detect that Pin.cxx was modified. I think there is some other place I should modify, but I didn't find it.