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 */
og/gnu/packages/dc.scm'>log</a><a href='/guix/tree/gnu/packages/dc.scm?id=8b6b097714962beeb675cad6425b2417dc564dab'>tree</a><a href='/guix/commit/gnu/packages/dc.scm?id=8b6b097714962beeb675cad6425b2417dc564dab'>commit</a><a href='/guix/diff/gnu/packages/dc.scm?id=8b6b097714962beeb675cad6425b2417dc564dab'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/gnu/packages/dc.scm'>
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<div class='path'>path: <a href='/guix/log/?id=8b6b097714962beeb675cad6425b2417dc564dab'>root</a>/<a href='/guix/log/gnu?id=8b6b097714962beeb675cad6425b2417dc564dab'>gnu</a>/<a href='/guix/log/gnu/packages?id=8b6b097714962beeb675cad6425b2417dc564dab'>packages</a>/<a href='/guix/log/gnu/packages/dc.scm?id=8b6b097714962beeb675cad6425b2417dc564dab'>dc.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/dc.scm?id=8b6b097714962beeb675cad6425b2417dc564dab&amp;showmsg=1'>Expand</a>)</th><th class='left'>Author</th></tr>
<tr><td><span title='2021-12-13 17:48:25 +0100'>2021-12-13</span></td><td><a href='/guix/commit/gnu/packages/dc.scm?id=8394619baceb118df92e355377fd543bb1aa501a'>gnu: Simplify package inputs.</a><span class='msg-avail'>...<span class='msg-tooltip'>This commit was obtained by running:

  ./pre-inst-env guix style

without any additional argument.
</span></span></td><td>Ludovic Courtès</td></tr>
just for new version.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Timotej Lazar</td></tr>
<tr><td><span title='2023-05-19 16:46:34 +0200'>2023-05-19</span></td><td><a href='/guix/commit/gnu/packages/sssd.scm?id=f1400fe08fcb2500028f83df999d5e9c7f67f219'>gnu: sssd: Fix build</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sssd.scm (sssd) [native-inputs]: Add python-toolchain.
[ensure-no-mtimes-pre-1980]: New phase.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Brian Cully</td></tr>
<tr><td><span title='2023-05-19 16:46:34 +0200'>2023-05-19</span></td><td><a href='/guix/commit/gnu/packages/sssd.scm?id=379432f410e868eaf7a4937ecebd9bd16ffecab0'>gnu: sssd: Fix failing tests by using samba/pinned input</a><span class='msg-avail'>...<span class='msg-tooltip'>The latest samba package (4.18.1) causes a SIGABRT in ad_common_tests and
ad_gpo_tests. The pinned version (4.17.0) allows them to succeed, so use that
instead.

* gnu/packages/sssd.scm (sssd) [inputs]: Change samba to samba/pinned.

Signed-off-by: Ludovic Courtès &lt;ludo@gnu.org&gt;
</span></span></td><td>Brian Cully</td></tr>
<tr><td><span title='2023-04-21 00:44:07 -0400'>2023-04-21</span></td><td><a href='/guix/commit/gnu/packages/sssd.scm?id=09cc2636ef46d96df5a83e9034083e79e02a049b'>gnu: adcli: Add zlib.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sssd.scm (adcli) [inputs]: Add zlib.
</span></span></td><td>Maxim Cournoyer</td></tr>
<tr><td><span title='2023-04-21 00:44:07 -0400'>2023-04-21</span></td><td><a href='/guix/commit/gnu/packages/sssd.scm?id=8f0a110503a1936d60f24feec2ebdcb89e502b91'>gnu: adcli: Delete use-local-docbook phase.</a><span class='msg-avail'>...<span class='msg-tooltip'>* gnu/packages/sssd.scm (adcli) [arguments]: Delete use-local-docbook phase.
[native-inputs]: Replace docbook-xml with docbook-xml-4.3.  Add libxml2.
</span></span></td><td>Maxim Cournoyer</td></tr>