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);
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<div class='path'>path: <a href='/guix/log/?id=3546e97f38bb4dfa4c016cd7a20d4add277ca8f7'>root</a>/<a href='/guix/log/gnu?id=3546e97f38bb4dfa4c016cd7a20d4add277ca8f7'>gnu</a>/<a href='/guix/log/gnu/packages?id=3546e97f38bb4dfa4c016cd7a20d4add277ca8f7'>packages</a>/<a href='/guix/log/gnu/packages/libsigsegv.scm?id=3546e97f38bb4dfa4c016cd7a20d4add277ca8f7'>libsigsegv.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/libsigsegv.scm?id=3546e97f38bb4dfa4c016cd7a20d4add277ca8f7&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2018-05-07 00:11:19 +0200'>2018-05-07</span></td><td><a href='/guix/commit/gnu/packages/libsigsegv.scm?id=3cc9a8a1324b865fa70d61647c5a41de1e439522'>gnu: libsigsegv: Install the shared library.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2018-02-20 19:54:25 +0100'>2018-02-20</span></td><td><a href='/guix/commit/gnu/packages/libsigsegv.scm?id=3b1b5042298dfb8cc24d9f73c0fae3c9bdb09a7b'>gnu: libsigsegv: Update to 2.12.</a><span class='msg-avail'>...</span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2017-04-18 06:36:39 +0300'>2017-04-18</span></td><td><a href='/guix/commit/gnu/packages/libsigsegv.scm?id=0c1a9438dd64137a68481c64ae6b61f4660d536a'>gnu: libsigsegv: Use 'modify-phases' syntax.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-04-18 06:36:38 +0300'>2017-04-18</span></td><td><a href='/guix/commit/gnu/packages/libsigsegv.scm?id=74f04d85566c8a6a83ec06e60df1f5942fa853d8'>gnu: libsigsegv: Update to 2.11.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2017-03-30 01:30:16 +0200'>2017-03-30</span></td><td><a href='/guix/commit/gnu/packages/libsigsegv.scm?id=6fd52309b8f52c9bb59fccffac53e029ce94b698'>gnu: Use HTTPS for almost all gnu.org HOME-PAGEs.</a><span class='msg-avail'>...</span></td><td>Tobias Geerinckx-Rice</td></tr>