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-5.9.4.orig/src/corelib/tools/qtimezoneprivate_tz.cpp
+++ qtbase-opensource-src-5.9.4/src/corelib/tools/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=450dcd1aff741c4f81cc9508ce33f19e62edefb7'>root</a>/<a href='/guix/log/gnu?id=450dcd1aff741c4f81cc9508ce33f19e62edefb7'>gnu</a>/<a href='/guix/log/gnu/packages?id=450dcd1aff741c4f81cc9508ce33f19e62edefb7'>packages</a>/<a href='/guix/log/gnu/packages/screen.scm?id=450dcd1aff741c4f81cc9508ce33f19e62edefb7'>screen.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/packages/screen.scm?id=450dcd1aff741c4f81cc9508ce33f19e62edefb7&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2020-07-12 23:08:15 +0200'>2020-07-12</span></td><td><a href='/guix/commit/gnu/packages/screen.scm?id=b0e7b6992f3f845e83cfbca4d700b51dba50b4d5'>gnu: Remove ".git" from "https://github/…/….git".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-06-02 18:06:04 +0200'>2020-06-02</span></td><td><a href='/guix/commit/gnu/packages/screen.scm?id=482b8cb555069866bdfabdef2a026f1cd651d7a9'>gnu: byobu: Update to 5.133.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2020-03-26 12:59:34 +0100'>2020-03-26</span></td><td><a href='/guix/commit/gnu/packages/screen.scm?id=3da54cc3180ce175b1820dd4e366abdbbffcb45d'>gnu: screen: Build fix for the Hurd.</a><span class='msg-avail'>...</span></td><td>Jan Nieuwenhuizen</td></tr>
<tr><td><span title='2020-02-06 10:48:18 +0200'>2020-02-06</span></td><td><a href='/guix/commit/gnu/packages/screen.scm?id=45088584aca4ffc0e07215aa8c112a930374f57d'>gnu: screen: Update to 4.8.0.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2019-10-02 20:57:46 +0300'>2019-10-02</span></td><td><a href='/guix/commit/gnu/packages/screen.scm?id=dde9d858181c45dbed09a51145da4e66fc559997'>gnu: screen: Update to 4.7.0.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>