[i2c] py-smbus needs i2c-dev.h to be installed to build

[Jean Delvare]

Hi Mark,

As I moved py-smbus to the new i2c-tools package, I noticed that
building it only works if the i2c-dev.h header file is already
installed on the system. This doesn't sound right. I believe that one
should be able to build py-smbus before the i2c-tools package is
installed. What do you think?

In order to let py-smbus build in place, we would need to add
-I../include to the gcc options, but I have no idea how to do this.

Thanks,
-- 
Jean Delvare