Look for input_files also in output directory
config_p: don't overwrite function ref with result

We were replacing the function reference whith it's return value.
This doesn't work when the function can return different results for
different projects.
Cache result from lsb_release

Cache result from lsb_release for better performance. This function is
used for all calls to project_config (to get the running distro, which
can influence the result), so it needs to be fast.
input_files: print when a build is finished