Use $TZDIR to search for time-zone data. Thus avoid depending on package
"tzdata", which often introduces changes with near-immediate effects, so it's
important to be able to update it fast.

Based on a patch fron NixOS.
===================================================================
--- qtbase-opensource-src-5.14.2.orig/src/corelib/time/qtimezoneprivate_tz.cpp
+++ qtbase-opensource-src-5.15.2/src/corelib/time/qtimezoneprivate_tz.cpp
@@ -70,7 +70,11 @@
 // Parse zone.tab table, assume lists all installed zones, if not will need to read directories
 static QTzTimeZoneHash loadTzTimeZones()
 {
-    QString path = QStringLiteral("/usr/share/zoneinfo/zone.tab");
+    // Try TZDIR first, in case we're running on GuixSD.
+    QString path = QFile::decodeName(qgetenv("TZDIR")) + QStringLiteral("/zone.tab");
+    // Fallback to traditional paths in case we are not on GuixSD.
+    if (!QFile::exists(path))
+        path = QStringLiteral("/usr/share/zoneinfo/zone.tab");
     if (!QFile::exists(path))
         path = QStringLiteral("/usr/lib/zoneinfo/zone.tab");
 
@@ -645,6 +649,9 @@
         if (!tzif.open(QIODevice::ReadOnly))
             return;
     } else {
+      // Try TZDIR first, in case we're running on GuixSD.
+      tzif.setFileName(QFile::decodeName(qgetenv("TZDIR")) + QStringLiteral("/") + QString::fromLocal8Bit(ianaId));
+      if (!tzif.open(QIODevice::ReadOnly)) {
         // Open named tz, try modern path first, if fails try legacy path
         tzif.setFileName(QLatin1String("/usr/share/zoneinfo/") + QString::fromLocal8Bit(ianaId));
         if (!tzif.open(QIODevice::ReadOnly)) {
@@ -652,6 +659,7 @@
             if (!tzif.open(QIODevice::ReadOnly))
                 return;
         }
+      }
     }
 
     QDataStream ds(&tzif);
option value='author'>author</option>
<option value='committer'>committer</option>
<option value='range'>range</option>
</select>
<input class='txt' type='search' size='10' name='q' value=''/>
<input type='submit' value='search'/>
</form>
</td></tr></table>
<div class='path'>path: <a href='/guix/log/?id=4e1f7b1df67955c6ebd27bf83a07933ca521b174'>root</a>/<a href='/guix/log/gnu?id=4e1f7b1df67955c6ebd27bf83a07933ca521b174'>gnu</a>/<a href='/guix/log/gnu/services?id=4e1f7b1df67955c6ebd27bf83a07933ca521b174'>services</a>/<a href='/guix/log/gnu/services/sysctl.scm?id=4e1f7b1df67955c6ebd27bf83a07933ca521b174'>sysctl.scm</a></div><div class='content'><table class='list nowrap'><tr class='nohover'><th class='left'>Age</th><th class='left'>Commit message (<a href='/guix/log/gnu/services/sysctl.scm?id=4e1f7b1df67955c6ebd27bf83a07933ca521b174&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-04-29 18:07:16 +0200'>2022-04-29</span></td><td><a href='/guix/commit/gnu/services/sysctl.scm?id=9d7248cd87fbda8c717341a8092780782bef0820'>services: Add missing 'description' fields.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/databases.scm (postgresql-service-type)[description]: New field.
(memcached-service-type)[description]: New field.
(mysql-service-type)[description]: New field.
(redis-service-type)[description]: New field.
* gnu/services/desktop.scm (geoclue-service-type)[description]: New
field.
(udisks-service-type)[description]: New field.
(elogind-service-type)[description]: New field.
(account-service-type)[description]: New field.
* gnu/services/kerberos.scm (krb5-service-type)[description]: New field.
(pam-krb5-service-type)[description]: New field.
* gnu/services/lirc.scm (lirc-service-type)[description]: New field.
* gnu/services/mail.scm (dovecot-service-type)[description]: New field.
(opensmtpd-service-type)[description]: New field.
(mail-aliases-service-type)[description]: New field.
(exim-service-type)[description]: New field.
* gnu/services/monitoring.scm (zabbix-server-service-type)[description]:
New field.
(zabbix-agent-service-type)[description]: New field.
* gnu/services/nfs.scm (rpcbind-service-type)[description]: New field.
(pipefs-service-type)[description]: New field.
(gss-service-type)[description]: New field.
(idmap-service-type)[description]: New field.
* gnu/services/spice.scm (spice-vdagent-service-type)[description]: New field.
* gnu/services/sysctl.scm (sysctl-service-type)[description]: New field.
* gnu/services/virtualization.scm (libvirt-service-type)[description]:
New field.
(virtlog-service-type)[description]: New field.
* gnu/services/vpn.scm (openvpn-server-service-type)[description]: New field.
(openvpn-client-service-type)[description]: New field.
(wireguard-service-type)[description]: New field.
* gnu/services/web.scm (httpd-service-type)[description]: New field.
(fcgiwrap-service-type)[description]: New field.
(agate-service-type)[description]: New field.
[name]: Fix.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-03-31 15:36:49 +0200'>2021-03-31</span></td><td><a href='/guix/commit/gnu/services/sysctl.scm?id=4f547c50f9bc478fd7c8bb9d481e2e376b969e8a'>services: sysctl: Export &lt;sysctl-configuration&gt; record field accessors.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/services/sysctl.scm (sysctl-configuration-sysctl)
(sysctl-configuration-settings): Export.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>muradm</td></tr>
<tr><td><span title='2021-03-18 12:17:33 +0100'>2021-03-18</span></td><td><a href='/guix/commit/gnu/services/sysctl.scm?id=898489f48e436e45e86e1ba0fcdb6df5cd5a051a'>services: Enable "protected hardlinks" and "protected symlinks" by default.</a><span class='msg-avail'>...<span class='msg-tooltip'>References:

https://sysctl-explorer.net/fs/protected_hardlinks/
https://sysctl-explorer.net/fs/protected_symlinks/

* gnu/services/sysctl.scm (%default-sysctl-settings): New public variable.
(&lt;sysctl-configuration&gt;): Use %default-sysctl-settings as the default value.
* gnu/services/base.scm (%base-services): Add sysctl-service-type.
* doc/guix.texi (Miscellaneous Services): Document the new defaults.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Leo Famulari</td></tr>