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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<tr><td><span title='2019-03-21 04:06:58 +0100'>2019-03-21</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=e6b244a588ee69ca27dd54236728e0656f1326b6'>gnu: plantuml: Update to 1.2019.3.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/uml.scm (plantuml): Update to 1.2019.3.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-03-06 00:15:27 +0100'>2019-03-06</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=84bdd0ec06a87bcc63fbfa597bc8b386b79058d8'>gnu: plantuml: Update to 1.2019.2.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/uml.scm (plantuml): Update to 1.2019.2.
</span></span></td><td>Tobias Geerinckx-Rice</td></tr>
<tr><td><span title='2019-02-09 13:40:12 +0530'>2019-02-09</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=722ac64cd7dc1f09fb77e2ae780427fa13c03110'>gnu: plantuml: Update to 1.2019.0.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/uml.scm (plantuml): Update to 1.2019.0.
</span></span></td><td>Arun Isaac</td></tr>
<tr><td><span title='2017-10-03 17:07:54 +0200'>2017-10-03</span></td><td><a href='/guix/commit/gnu/packages/uml.scm?id=d109b1e85c5c1438e66dfc05ea4a018418edaf19'>Update e-mail for Theodoros Foradis.</a><span class='msg-avail'>...<span class='msg-tooltip'>* .mailmap: Add entry.
* gnu/packages/aspell.scm: Replace theodoros.for@openmailbox.org with current address.
* gnu/packages/compression.scm: Likewise.
* gnu/packages/electronics.scm: Likewise.
* gnu/packages/engineering.scm: Likewise.
* gnu/packages/fpga.scm: Likewise.
* gnu/packages/graphviz.scm: Likewise.
* gnu/packages/guile.scm: Likewise.
* gnu/packages/libusb.scm: Likewise.
* gnu/packages/maths.scm: Likewise.
* gnu/packages/messaging.scm: Likewise.
* gnu/packages/uml.scm: Likewise.
* gnu/packages/wxwidgets.scm: Likewise.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Theodoros Foradis</td></tr>