Fix build with GLib 2.68 and later.

Taken from upstream pull request:
https://github.com/autotrace/autotrace/pull/41

diff --git a/src/autotrace.h b/src/autotrace.h
--- a/src/autotrace.h
+++ b/src/autotrace.h
@@ -23,6 +23,9 @@
 
 #include <stdio.h>
 
+#include "types.h"
+#include "color.h"
+
 #ifdef __cplusplus
 extern "C" {
 #endif                          /* __cplusplus */
@@ -35,9 +38,6 @@ extern "C" {
  * Typedefs
  * ===================================================================== */
 
-#include "types.h"
-#include "color.h"
-
 /* Third degree is the highest we deal with.  */
   enum _at_polynomial_degree {
     AT_LINEARTYPE = 1,
diff --git a/src/color.h b/src/color.h
--- a/src/color.h
+++ b/src/color.h
@@ -24,6 +24,10 @@
 #include <glib.h>
 #include <glib-object.h>
 
+#ifdef __cplusplus
+extern "C" {
+#endif                          /* __cplusplus */
+
 typedef struct _at_color at_color;
 struct _at_color {
   guint8 r;
@@ -43,4 +47,7 @@ void at_color_free(at_color * color);
 GType at_color_get_type(void);
 #define AT_TYPE_COLOR (at_color_get_type ())
 
+#ifdef __cplusplus
+}
+#endif                          /* __cplusplus */
 #endif /* not AT_COLOR_H */
'/guix/log/gnu/packages/matrix.scm'>log</a><a href='/guix/tree/gnu/packages/matrix.scm?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>tree</a><a href='/guix/commit/gnu/packages/matrix.scm?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>commit</a><a href='/guix/diff/gnu/packages/matrix.scm?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/matrix.scm'>
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<div class='path'>path: <a href='/guix/log/?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>root</a>/<a href='/guix/log/gnu?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>gnu</a>/<a href='/guix/log/gnu/packages?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>packages</a>/<a href='/guix/log/gnu/packages/matrix.scm?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd'>matrix.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/gnu/packages/matrix.scm?id=d95c4f71d5d26875344d0c9dfbba72df926ab0bd&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2022-06-02 16:58:14 +0200'>2022-06-02</span></td><td><a href='/guix/commit/gnu/packages/matrix.scm?id=bf9da5ccc908376c84d0dfc7f1531fb479d23134'>gnu: Remove python-atomicwrites-1.4.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/python-xyz.scm (python-atomicwrites-1.4): Remove variable.
* gnu/packages/matrix.scm (python-matrix-nio)[propagated-inputs]: Replace
python-atomicwrites-1.4 with python-atomicwrites.
</span></span></td><td>Ricardo Wurmus</td></tr>
<tr><td><span title='2022-05-12 12:45:54 -0400'>2022-05-12</span></td><td><a href='/guix/commit/gnu/packages/matrix.scm?id=bfe7f4065095fde94a44b511ae74186a2dbe880f'>gnu: python-hypothesis-6.23: Update to 6.43.3.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/check.scm (python-hypothesis-6.23): Update to 6.43.3 and rename
to...
(python-hypothesis-next): ... this.
[propagated-inputs]: Add python-pytest.
* gnu/packages/matrix.scm (python-matrix-nio): Adjust accordingly.
* gnu/packages/python-web.scm (python-h2): Likewise.
* gnu/packages/check.scm (python-hypothesis)[home-page]: Update URL.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2022-01-17 00:18:23 +0100'>2022-01-17</span></td><td><a href='/guix/commit/gnu/packages/matrix.scm?id=863b7b7dd7059e9e2c78f89b501919042159eccd'>gnu: pantalaimon: Add needed inputs</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/matrix.scm (pantalaimon)
&lt;propagated-inputs&gt;: Add python-dbus, python-notify2, python-pydbus, and
python-pygobject.
&lt;arguments&gt;: Run sanity-check phase.

Co-authored-by: Aleksandr Vityazev &lt;avityazev@posteo.org&gt;
Signed-off-by: Nicolas Goaziou &lt;mail@nicolasgoaziou.fr&gt;
</span></span></td><td>Morgan Smith</td></tr>
<tr><td><span title='2022-01-12 23:50:15 +0100'>2022-01-12</span></td><td><a href='/guix/commit/gnu/packages/matrix.scm?id=fb6de84a6b9ed38fc4fbf4fad89a0f8fe25111eb'>gnu: python-matrix-synapse-ldap3: Fix build.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/matrix.scm (python-matrix-synapse-ldap3)[arguments]: Delete
sanity check.
</span></span></td><td>Marius Bakke</td></tr>
<tr><td><span title='2022-01-11 18:36:38 +0100'>2022-01-11</span></td><td><a href='/guix/commit/gnu/packages/matrix.scm?id=6a86d59e28250771a3a0537108869f8c5bdee0a4'>gnu: pantalaimon: Disable sanity check.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/matrix.scm (pantalaimon)[arguments]: Disable sanity-check phase.
</span></span></td><td>Marius Bakke</td></tr>