no automatic rebuild when changing Buildfile

Buildr does not automatically rebuild components for which the build instructions in the Buildfile have changed. This leads to slight confusions when adjusting the Buildfile to get the desired results (at least for new users). Compare for example to CMake which does rebuild as necessary.

The quickest fix might be to cache a checksum or timestamp of the buildfile and clean before building whenever it changes (I think this is roughly what cmake does).