Improve performance a bit with GNU Make.

* Makefile (.PHONY): New rule, listing non-file targets.
Allow non-ASCII characters in the commentary.

* Makefile (INVALID_CHAR1): Rename to SAFE_CHARSET1.
(INVALID_CHAR2): Rename to SAFE_CHARSET2.
(INVALID_CHAR3): Rename to SAFE_CHARSET3.
(INVALID_CHAR): Remove.
(SAFE_CHARSET, SAFE_CHAR, SAFE_LINE, SAFE_SHARP_LINE, VALID_LINE):
New macros.
(check_character_set): Allow any UTF-8 character in some contexts,
including comments in the data, and in some auxiliary commentary files.
Change web page encoding from US-ASCII to UTF-8.

* Makefile (DOCS): Move $(WEB_PAGES) here ...
(MISC): ... from here, so that they can contain UTF-8.
* tz-art.htm, tz-link.htm: Switch to UTF-8 encoding.
Prefer UTF-8 to HTML entities, e.g., prefer '–' to '&ndash;'.
Now that we can use UTF-8, restore accents that ASCII lost.

* NEWS, Theory, africa, europe, northamerica, southamerica:
Don't insist on ASCII when normal English usage would have
accented words.
Fix Finland's 1942 fall-back transition.

* europe (Finland), NEWS: Finland's 1942 fall-back transition was
10-04 01:00, not 10-03 00:00.  (Thanks to Konstantin Hyppönen.)