* aclocal.in (install_file): Cannot use /dev/null while diffing

* aclocal.in (install_file): Cannot use /dev/null while diffing
new files, because Tru64's diff do not handle /dev/null.  So
create an empty destination file before running diff on a new
file, and erase it afterward.  Fall back to using /dev/null only
if we cannot create this file.
Report and initial patch from Ralf Wildenhues.
(unlink_tmp): New function.
* test/acloca18.test: Make sure the empty file has been erased.