Try to update config.cache atomically; respect symlinks.

Try to update config.cache atomically; respect symlinks.

* lib/autoconf/general.m4 (AC_CACHE_SAVE): Use `mv -f' to update
the cache file if it is a regular file and not a symlink.  Move
first to temporary name in the target directory if not in the
current directory for atomicity across mount points.
* tests/base.at (AC_CACHE_CHECK): Try symlinked cache file.
* doc/autoconf.texi (Cache Files): Leftover temporary cache
files may be deleted by the user.
* NEWS: Update.

Signed-off-by: Ralf Wildenhues <Ralf.Wildenhues@gmx.de>