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=d2df6e4d23add9cb7abfd4e61e6b4dbccb286ae1'>root</a>/<a href='/guix/log/tests?id=d2df6e4d23add9cb7abfd4e61e6b4dbccb286ae1'>tests</a>/<a href='/guix/log/tests/inferior.scm?id=d2df6e4d23add9cb7abfd4e61e6b4dbccb286ae1'>inferior.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/inferior.scm?id=d2df6e4d23add9cb7abfd4e61e6b4dbccb286ae1&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-07-08 13:51:34 +0100'>2022-07-08</span></td><td><a href='/guix/commit/tests/inferior.scm?id=b4c4a6acb1204ee53e95744236ee89985db32f91'>guix: inferior: Fix the behaviour of open-inferior #:error-port.</a></td><td>Christopher Baines</td></tr>
<tr><td><span title='2022-05-20 18:46:07 +0200'>2022-05-20</span></td><td><a href='/guix/commit/tests/inferior.scm?id=a4994d739306abcf3f36706012fb88b35a970e6b'>inferior: Close duplicate socketpair file descriptor.</a></td><td>Ludovic Courtès</td></tr>