On Linux, use /usr/share/zoneinfo instead of bundled database

It is unfortunately necessary for the timezone information to
occasionally be updated urgently -- for example the government of Russia
passed a law in July 2014 causing timezone changes in October 2014.
These changes are still not represented in the HHVM bundled database,
see https://phabricator.wikimedia.org/T74556 .

The distros have long dealt with this by patching PHP to get the timezone
database from the system, instead of using the bundled binary. The patch
from php5_5.5.9+dfsg-1ubuntu4.5 happens to apply cleanly to HHVM's copy
of timelib, and compiles correctly if one replaces php_scandir() with
scandir(). In PHP on Linux, php_scandir is an alias for scandir.

The patch was originally by Joe Orton of Red Hat:
http://thread.gmane.org/gmane.comp.php.devel/47609

It has since been maintained by Red Hat and regularly copied into
Debian and Ubuntu. The Ubuntu package carries this copyright notice:

patch: use_embedded_timezonedb.patch
contributor: Joe Orton <jorton@redhat.com>
copyright © 2008 Red Hat, Inc.
may be used/modified/redistributed under the terms of PHP itself

The CMakeLists.txt change is my own.

This change is submitted on behalf of a third party, see above for
details.