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

diff -ru gobject-introspection-1.58.1.orig/giscanner/__init__.py gobject-introspection-1.58.1/giscanner/__init__.py
--- gobject-introspection-1.58.1.orig/giscanner/__init__.py	1970-01-01 01:00:00.000000000 +0100
+++ gobject-introspection-1.58.1/giscanner/__init__.py	2018-12-03 13:33:28.788971299 +0100
@@ -22,6 +22,8 @@
 builddir = os.environ.get('UNINSTALLED_INTROSPECTION_BUILDDIR')
 if builddir is not None:
     __path__.append(os.path.join(builddir, 'giscanner'))
+if not 'CC' in os.environ:
+    os.environ['CC'] = 'gcc'
 try:
     from ._version import __version__
 except ImportError:
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<tr><td><span title='2020-05-22 00:35:23 +0200'>2020-05-22</span></td><td><a href='/guix/commit/build-aux/check-channel-news.scm?id=1ad5209d904d471ded6cf53b4e29b64e963dea3f'>maint: Add "make check-channel-news".</a><span class='msg-avail'>...</span></td><td>Ludovic Courtès</td></tr>