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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ref='/guix/commit/gnu/packages/aux-files/guile-launcher.c?id=d92ee0a8bdc324726e737bf4ef099d75724ce8c9'>guile: Leave 'scm_install_gmp_memory_functions' unchanged.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2021-02-07 22:28:55 +0100'>2021-02-07</span></td><td><a href='/guix/commit/gnu/packages/aux-files/guile-launcher.c?id=a53f711422f63d7e32b8639b968cf00bcc69ffea'>guile: Do not install Guile/libgc allocators for GMP.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-12-11 19:06:53 +0100'>2020-12-11</span></td><td><a href='/guix/commit/gnu/packages/aux-files/guile-launcher.c?id=d20665ecbfe7278f550a1ed4cd76565013feab59'>guix: 'guile' executable ignores GUILE_LOAD_PATH during startup.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>
<tr><td><span title='2020-10-05 23:19:19 +0200'>2020-10-05</span></td><td><a href='/guix/commit/gnu/packages/aux-files/guile-launcher.c?id=1d4ab335b22a93e01c2eb1eb3e93fc6534157040'>self: Use a 'guile' that doesn't complain about locales.</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>