Use gcc as the default C compiler if CC is not set.

diff --git a/giscanner/__init__.py b/giscanner/__init__.py
index 7c2f365a..607fe341 100644
--- a/giscanner/__init__.py
+++ b/giscanner/__init__.py
@@ -21,6 +21,8 @@ import os
 builddir = os.environ.get('UNINSTALLED_INTROSPECTION_BUILDDIR')
 if builddir is not None:
     __path__.append(os.path.join(builddir, 'giscanner'))  # type: ignore  # mypy issue #1422
+if not 'CC' in os.environ:
+    os.environ['CC'] = 'gcc'
 try:
     from ._version import __version__
 except ImportError:
ass='form'><form method='get'>
<input type='hidden' name='id' value='843bec9a15eb3f3293ddfeefde97bde461ced7ee'/><select name='h' onchange='this.form.submit();'>
<option value='koszko' selected='selected'>koszko</option>
<option value='koszko-scripts'>koszko-scripts</option>
</select> <input type='submit' value='switch'/></form></td></tr>
<tr><td class='sub'>Wojtek's customized Guix</td><td class='sub right'></td></tr></table>
<table class='tabs'><tr><td>
<a href='/guix/about/'>about</a><a href='/guix/'>summary</a><a href='/guix/refs/?id=843bec9a15eb3f3293ddfeefde97bde461ced7ee'>refs</a><a class='active' href='/guix/log/etc/teams/rust'>log</a><a href='/guix/tree/etc/teams/rust?id=843bec9a15eb3f3293ddfeefde97bde461ced7ee'>tree</a><a href='/guix/commit/etc/teams/rust?id=843bec9a15eb3f3293ddfeefde97bde461ced7ee'>commit</a><a href='/guix/diff/etc/teams/rust?id=843bec9a15eb3f3293ddfeefde97bde461ced7ee'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/teams/rust'>
<input type='hidden' name='id' value='843bec9a15eb3f3293ddfeefde97bde461ced7ee'/><select name='qt'>
<option value='grep'>log msg</option>
<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=843bec9a15eb3f3293ddfeefde97bde461ced7ee'>root</a>/<a href='/guix/log/etc?id=843bec9a15eb3f3293ddfeefde97bde461ced7ee'>etc</a>/<a href='/guix/log/etc/teams?id=843bec9a15eb3f3293ddfeefde97bde461ced7ee'>teams</a>/<a href='/guix/log/etc/teams/rust?id=843bec9a15eb3f3293ddfeefde97bde461ced7ee'>rust</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/etc/teams/rust?id=843bec9a15eb3f3293ddfeefde97bde461ced7ee&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2024-11-28 11:05:02 +0200'>2024-11-28</span></td><td><a href='/guix/commit/etc/teams/rust?id=00acb49a9956231bc589bba92c22c4add0c9565b'>teams: rust: Add another manifest.</a><span class='msg-avail'>...<span class='msg-tooltip'>Change-Id: Icabed521eb71fa94237246b181974716bc8a3858
</span></span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2024-01-04 12:31:45 +0200'>2024-01-04</span></td><td><a href='/guix/commit/etc/teams/rust?id=8b559b37eafbf51fe4a602c091754a38b8ad28f7'>etc: Add rust-team manifest.</a><span class='msg-avail'>...<span class='msg-tooltip'>Change-Id: Ie43bd4a88d35270dcac7baaabeafbcc3a755db1e
</span></span></td><td>Efraim Flashner</td></tr>