Scons: If provided by environment variable use clcache.
Scons: If provided by environment variable use clcache.

* Also cleanup how this is integrated with cache mode of scons,
  avoiding to cache in scons, if an external caching is going
  on.

kayhayen/Nuitka,kayhayen/Nuitka,kayhayen/Nuitka,kayhayen/Nuitka