build: port to MinGW and fix symlink bugs

MinGW problem reported by Theo Veenker in
<http://mm.icann.org/pipermail/tz/2013-September/020378.html>.
The patched code should work even on POSIXish hosts
where 'link' and 'symlink' fail for whatever reason,
e.g., if the destination file system is VFAT.
Also, the patch fixes some bugs when symlinks are used.
* Makefile (CFLAGS): Mention HAVE_LINK in the comment.
(TZDSRCS): Add asctime.o, since MinGW lacks asctime.
(TZDOBJS): Add asctime.c.
* private.h (HAVE_LINK): Default to 1.
* zic.c (link) [!HAVE_LINK]: New macro.
(symlink) [!HAVE_SYMLINK]: New macro.
(dolink): Fix some bugs when 'link' fails but 'symlink' succeeds.
Fall back on copying when both fail.
* NEWS: Document the above.