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:
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<div class='path'>path: <a href='/guix/log/?id=0fbb356714f4ae3ea18e4997131a0e1746cc923c'>root</a>/<a href='/guix/log/tests?id=0fbb356714f4ae3ea18e4997131a0e1746cc923c'>tests</a>/<a href='/guix/log/tests/lint.scm?id=0fbb356714f4ae3ea18e4997131a0e1746cc923c'>lint.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/lint.scm?id=0fbb356714f4ae3ea18e4997131a0e1746cc923c&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2023-03-28 22:22:50 -0400'>2023-03-28</span></td><td><a href='/guix/commit/tests/lint.scm?id=cc7a79a8ade673cc66249c7b4583a0074348b370'>Revert "lint: Append "/info/refs" to git-reference-url."</a><span class='msg-avail'>...</span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2023-03-27 18:48:53 +0200'>2023-03-27</span></td><td><a href='/guix/commit/tests/lint.scm?id=c9af27d4ca733b20f09019f1465d3e5fdc1ec724'>lint: Append "/info/refs" to git-reference-url.</a><span class='msg-avail'>...</span></td><td>Felix Lechner</td></tr>
<tr><td><span title='2022-10-27 19:43:06 +0200'>2022-10-27</span></td><td><a href='/guix/commit/tests/lint.scm?id=c485a9d8af210425a0435280fff8588563e49684'>gnu: python-setuptools: Move to python-build.</a><span class='msg-avail'>...</span></td><td>Lars-Dominik Braun</td></tr>
<tr><td><span title='2022-10-17 23:15:08 +0200'>2022-10-17</span></td><td><a href='/guix/commit/tests/lint.scm?id=2383e145185efb2e6f99931707ec93d65d166432'>lint: source: Add check for &lt;svn-reference&gt; over HTTP(S).</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2022-03-29 14:43:14 +0300'>2022-03-29</span></td><td><a href='/guix/commit/tests/lint.scm?id=67213a27592122e05c196c490de2ddf990d6832a'>download: Use https for main KDE source.</a><span class='msg-avail'>...</span></td><td>Efraim Flashner</td></tr>
<tr><td><span title='2022-03-11 23:34:47 -0500'>2022-03-11</span></td><td><a href='/guix/commit/tests/lint.scm?id=88e44f7e1615579cee27a08bcde31248e65c1178'>lint: check-tests-true: Allow #:tests? #t for some build systems.</a><span class='msg-avail'>...</span></td><td>Maxime Devos</td></tr>
<tr><td><span title='2021-12-17 21:04:25 -0800'>2021-12-17</span></td><td><a href='/guix/commit/tests/lint.scm?id=5f547a5c425cc99553ea713703e09a8db9f3c38b'>lint: Adjust patch file length check.</a><span class='msg-avail'>...</span></td><td>Vagrant Cascadian</td></tr>