configure.ac: Comment out release timestamp.

Set related environment vars only if timestamp is set.
utility.cpp: Add "pre-" or "pre-release" if release
timestamp is not set.  Don't print empty timestamp.

git-svn-id: https://svn.code.sf.net/p/smartmontools/code/trunk@5341 4ea69e1a-61f1-4043-bf83-b5c94c648137