AtlasBuildScripts: fix bug in logic to decide on externals rebuild
Rewrite the code to decide if an externals rebuild is needed. Instead of using a combination of the externals version number and a hash of all (c)make arguments, simply concatenate all relevant information into one big "stamp" file.
This fixes also the problem of re-using stale builds (ATLINFR-5026).
Relates to ATLINFR-3018.
Edited by Frank Winklmeier