New alias Europe/Busingen for Europe/Zurich.

* europe, zone.tab (Europe/Busingen): New Link.  See Arthur David Olson
in <http://mm.icann.org/pipermail/tz/2013-January/018511.html>.
Original heads-up from Tobias Conradi in
<http://mm.icann.org/pipermail/tz/2011-September/008784.html>.
Move 'make check' to Ubuntu 12.10.

* Makefile (SGML_DTDDIR): New macro, for easier porting.
Update to file name used in Ubuntu 12.10.
(SGML_SEARCH_PATH): Use SGML_DTDDIR.