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);
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<div class='path'>path: <a href='/guix/log/?id=f0f53a817ab4c1d5052d12df9b9a7c6befb9cd65'>root</a>/<a href='/guix/log/tests?id=f0f53a817ab4c1d5052d12df9b9a7c6befb9cd65'>tests</a>/<a href='/guix/log/tests/gexp.scm?id=f0f53a817ab4c1d5052d12df9b9a7c6befb9cd65'>gexp.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/tests/gexp.scm?id=f0f53a817ab4c1d5052d12df9b9a7c6befb9cd65&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-03-30 22:48:44 +0200'>2021-03-30</span></td><td><a href='/guix/commit/tests/gexp.scm?id=da86e90efe796ee68ee0a5ccaf46f45cadb52336'>gexp: Add 'sexp-&gt;gexp'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/gexp.scm (sexp-&gt;gexp): New procedure.
* tests/gexp.scm ("sexp-&gt;gexp"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-03-03 14:19:25 +0100'>2021-03-03</span></td><td><a href='/guix/commit/tests/gexp.scm?id=9fc4e94986e68e0e33b260e2389765e2d3b7dd07'>gexp: #:references-graphs refers to non-native derivations.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes a regression introduced in
c6d6aee6659acb293eb33f498fdac3b47a19a48, where #:reference-graphs would
end up referring to native inputs.

This would notably break the compilation of systems using a childhurd,
because they would attempt to build the 'hurd' package natively.

* guix/gexp.scm (lower-reference-graphs)[tuple-&gt;gexp-input]: Honor TARGET.
* tests/gexp.scm ("gexp-&gt;derivation #:references-graphs cross-compilation"):
New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-02-23 15:24:48 +0100'>2021-02-23</span></td><td><a href='/guix/commit/tests/gexp.scm?id=b57de6fea126f907a873ae14ad8b32dc32456e8e'>gexp: Micro-optimize 'gexp-&gt;sexp' and 'lower-inputs'.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/gexp.scm (lower-inputs, gexp-&gt;sexp): Change keyword parameters to
positional parameters.  Adjust callers accordingly.
* tests/gexp.scm (gexp-&gt;sexp*, "gexp-&gt;file"): Adjust accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-02-23 15:24:48 +0100'>2021-02-23</span></td><td><a href='/guix/commit/tests/gexp.scm?id=4fa9d48fd47df45372fddf2251c3fc0afd48fda0'>gexp: 'gexp-inputs' returns both native and non-native inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>This avoids double traversal of references and extra bookkeeping,
thereby further reducing memory allocations.

* guix/gexp.scm (lower-gexp): Include only one call to 'lower-inputs'.
(gexp-inputs): Remove #:native? parameter.
[set-gexp-input-native?]: New procedure.
[add-reference-inputs]: Use it.
(gexp-native-inputs): Remove.
* tests/gexp.scm (gexp-native-inputs): Remove.
(gexp-input-&gt;tuple): Include 'gexp-input-native?'.
("let-system")
("let-system, nested")
("ungexp + ungexp-native")
("ungexp + ungexp-native, nested")
("ungexp + ungexp-native, nested, special mixture")
("input list")
("input list + ungexp-native")
("input list splicing")
("input list splicing + ungexp-native-splicing")
("gexp list splicing + ungexp-splicing"): Adjust accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-02-23 15:24:48 +0100'>2021-02-23</span></td><td><a href='/guix/commit/tests/gexp.scm?id=fc6d6aee6659acb293eb33f498fdac3b47a19a48'>gexp: 'gexp-inputs' returns a list of &lt;gexp-input&gt; records.</a><span class='msg-avail'>...<span class='msg-tooltip'>This slightly reduces memory allocation.

* guix/gexp.scm (lower-inputs): Expect a list of &lt;gexp-input&gt; rather
than a list of tuples.
(lower-reference-graphs)[tuple-&gt;gexp-input]: New procedure.
Use it.
(gexp-inputs): Return a list of &lt;gexp-input&gt; rather than a list of
tuples.
* tests/gexp.scm (gexp-input-&gt;tuple): New procedure.
("one input package")
("one input package, dotted list")
("one input origin")
("one local file")
("one local file, symlink")
("one plain file")
("two input packages, one derivation, one file")
("file-append")
("file-append, output")
("file-append, nested")
("let-system")
("let-system, nested")
("ungexp + ungexp-native")
("ungexp + ungexp-native, nested")
("ungexp + ungexp-native, nested, special mixture")
("input list")
("input list + ungexp-native")
("input list splicing")
("input list splicing + ungexp-native-splicing")
("gexp list splicing + ungexp-splicing"): Adjust accordingly.
</span></span></td><td>Ludovic Courtès</td></tr>