On Guix, Python modules are in a different output from the executables,
so searching "../share/glib-2.0" will not work.

This patch restores behaviour prior to this commit:
<https://git.gnome.org/browse/glib/commit/?id=fe2a9887a8ccb14f2386e01b14834e97a33bc2d7>

--- a/gio/gdbus-2.0/codegen/gdbus-codegen.in
+++ b/gio/gdbus-2.0/codegen/gdbus-codegen.in
@@ -25,9 +25,12 @@
 
 srcdir = os.getenv('UNINSTALLED_GLIB_SRCDIR', None)
 filedir = os.path.dirname(__file__)
+datadir = os.path.join('@datadir@', 'glib-2.0')
 
 if srcdir is not None:
     path = os.path.join(srcdir, 'gio', 'gdbus-2.0')
+elif os.path.exists(os.path.join(datadir, 'codegen')):
+    path = datadir
 elif os.path.basename(filedir) == 'bin':
     # Make the prefix containing gdbus-codegen 'relocatable' at runtime by
     # adding /some/prefix/bin/../share/glib-2.0 to the python path
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<div class='path'>path: <a href='/guix/log/?id=a40af5d0ad707906815ffdd48bd2aec89b4cc4e9'>root</a>/<a href='/guix/log/gnu?id=a40af5d0ad707906815ffdd48bd2aec89b4cc4e9'>gnu</a>/<a href='/guix/log/gnu/tests?id=a40af5d0ad707906815ffdd48bd2aec89b4cc4e9'>tests</a>/<a href='/guix/log/gnu/tests/dict.scm?id=a40af5d0ad707906815ffdd48bd2aec89b4cc4e9'>dict.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/tests/dict.scm?id=a40af5d0ad707906815ffdd48bd2aec89b4cc4e9&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2017-07-20 11:57:13 +0200'>2017-07-20</span></td><td><a href='/guix/commit/gnu/tests/dict.scm?id=8b113790fa3bfd2300c737901ba161f079fedbdf'>tests: Use 'virtual-machine' records instead of monadic procedures.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests/base.scm (%test-basic-os): Use 'let*' instead of 'mlet*' and
'virtual-machine' instead of 'system-qemu-image/shared-store-script'.
(run-mcron-test): Likewise.
(run-nss-mdns-test): Likewise.
* gnu/tests/dict.scm (run-dicod-test): Likewise.
* gnu/tests/mail.scm (run-opensmtpd-test): Likewise.
(run-exim-test): Likewise.
* gnu/tests/messaging.scm (run-xmpp-test): Likewise.
* gnu/tests/networking.scm (run-inetd-test): Likewise.
* gnu/tests/nfs.scm (run-nfs-test): Likewise.
* gnu/tests/ssh.scm (run-ssh-test): Likewise.
* gnu/tests/web.scm (run-nginx-test): Likewise.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-04-16 00:48:08 +0200'>2017-04-16</span></td><td><a href='/guix/commit/gnu/tests/dict.scm?id=cbe0af11b2bffb1ef534941ac29d1384c9f09549'>tests: dicod: Bail out if we cannot connect within 20 seconds.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests/dict.scm (run-dicod-test)["connect inside"]: Bail out after
20 seconds of failure to connect.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2017-04-01 00:45:18 +0200'>2017-04-01</span></td><td><a href='/guix/commit/gnu/tests/dict.scm?id=985a8599ed742053b52ac85f753c4feb54af93cb'>tests: Add GNU dicod test.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/tests/dict.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
</span></span></td><td>Ludovic Courtès</td></tr>