Remove backup-files from the TARGET_DIR.

Closes #823.

~ is commonly used as a suffix to editor backup files.
Remove these files, just as we remove CVS-directories and
.empty-files.

[Peter: tweaked to remove in same find invocation as .empty]
Signed-off-by: Anders Darander <ad@datarespons.se>
Signed-off-by: Peter Korsgaard <jacmet@sunsite.dk>