Improvements:
* allow to mount a directory from worker's host OS as the /.ccache/ in the build root, so cache can be shared (including over NFS; tested for an ARM+AMD64 farm)
* use a /.ccache-bin/ rather than /var/lib/build/ccache/bin (easier to clean out)
* more reliably test for presence of compilers where absolute symlinks are used in build root (e.g. debian alternatives)
* allow fallback use of a ccache binary provided by the worker's host OS
* in case of cleanup, hit not only compiler basenames but also possible suffixes (arch, version, etc.)

If ccache was used in this build, optionally display its log in the end of build, so packagers know better if their recipes need to be tuned to take advantage of ccache (tune relative paths to source, etc.)
This should be enabled either via command-line arguments to "build", or caller can also enforce this via envvars (e.g. /etc/sysconfig/obs-worker file) with `export DO_CCACHE_STATISTICS=1`

…e ccache artefacts into produced images

…requirements

…STATS that can be used to enable details while sourcing configs (e.g. via worker setup)

…lso pre-create the /.build.log.ccache and chown it to abuild user

…cache file

…ilenames

…ame+version dir