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=ded6beb9a713c338327b008b02a4788ef40ea331'>root</a>/<a href='/guix/log/tests?id=ded6beb9a713c338327b008b02a4788ef40ea331'>tests</a>/<a href='/guix/log/tests/gnu-maintenance.scm?id=ded6beb9a713c338327b008b02a4788ef40ea331'>gnu-maintenance.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/gnu-maintenance.scm?id=ded6beb9a713c338327b008b02a4788ef40ea331&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-05-28 23:04:44 +0200'>2021-05-28</span></td><td><a href='/guix/commit/tests/gnu-maintenance.scm?id=d7c356edb9719f1e236ee926c0288f914076481a'>gnu-maintenance: 'release-file?' accepts 'v' prefix as in "PKG-v1.2.tgz".</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/gnu-maintenance.scm (%tarball-rx, %package-name-rx): Accept 'v'
and 'V' prefixes.  Accept ".tgz" extension.
* tests/gnu-maintenance.scm ("release-file?"): Add test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-04-05 12:03:09 +0200'>2021-04-05</span></td><td><a href='/guix/commit/tests/gnu-maintenance.scm?id=ceeea60bbc1e2be6a86cef208fcd80eb61c92934'>gnu-maintenance: Recognize more source tarball naming schemes.</a><span class='msg-avail'>...<span class='msg-tooltip'>* guix/gnu-maintenance.scm (%package-name-rx): Add ".src" and ".orig"
suffixes.
* tests/gnu-maintenance.scm ("release-file?"): Add mpg321 and bvi
examples.
("tarball-&gt;version"): New test.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-03-29 23:33:40 +0200'>2021-03-29</span></td><td><a href='/guix/commit/tests/gnu-maintenance.scm?id=1575da60628cf2594cd7b1ab0812ec4c50ac5092'>gnu-maintenance: Recognize "-source" tarball suffix.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://bugs.gnu.org/47398&gt;.
Reported by Léo Le Bouter &lt;lle-bout@zaclys.net&gt;.

* guix/gnu-maintenance.scm (%tarball-rx): Add "-[Ss]ource" suffix.
* tests/gnu-maintenance.scm ("release-file?"): Add exiv2 example.
</span></span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-03-20 22:29:14 +0100'>2021-03-20</span></td><td><a href='/guix/commit/tests/gnu-maintenance.scm?id=1710e8cb59e2ef6f1e0eb19235b6a4f1b8158bc4'>gnu-maintenance: Accept underscores as package/version separators.</a><span class='msg-avail'>...<span class='msg-tooltip'>Fixes &lt;https://bugs.gnu.org/47256&gt;.
Reported by Léo Le Bouter &lt;lle-bout@zaclys.net&gt;.

* guix/gnu-maintenance.scm (%tarball-rx, %package-name-rx): Accept
underscore as the package/version separator in tarball names.
* tests/gnu-maintenance.scm ("release-file?"): Add "mediainfo" test.
</span></span></td><td>Ludovic Courtès</td></tr>