Revert "only rebuild revno,.h if something has changed. Default to the date if GIT...

Revert "only rebuild revno,.h if something has changed. Default to the date if GIT isn't found."

This reverts commit fbc5d9e9d82f596a5872a4e162b65aa48428aefc.

The revert is due to misleading commit message:
 - revno.h was only rebuilt if sources changed before this commit, while
   this commit forces removal of it and thus unconditional re-creation
 - this commit moves revno.h from source tree to build tree w/out saying
   so in the commit log

Additionally (but would normally not trigger a revert)
 - the date default is useless as revno.h should always be in packages
   obtained w/out git and you can always generate if you got the sources
   using git.

NOTE: I wouldn't revert commits if they weren't themselves reverts of mine
      in the first place. Please discuss changes on the development mailing
      list to avoid commit wars...