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 */
35f406a86ed7b8'>refs</a><a class='active' href='/guix/log/etc/completion/zsh?showmsg=1'>log</a><a href='/guix/tree/etc/completion/zsh?id=ce451cdcb8f36ca48d4fe70f2235f406a86ed7b8'>tree</a><a href='/guix/commit/etc/completion/zsh?id=ce451cdcb8f36ca48d4fe70f2235f406a86ed7b8'>commit</a><a href='/guix/diff/etc/completion/zsh?id=ce451cdcb8f36ca48d4fe70f2235f406a86ed7b8'>diff</a></td><td class='form'><form class='right' method='get' action='/guix/log/etc/completion/zsh'>
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<div class='path'>path: <a href='/guix/log/?id=ce451cdcb8f36ca48d4fe70f2235f406a86ed7b8&amp;showmsg=1'>root</a>/<a href='/guix/log/etc?id=ce451cdcb8f36ca48d4fe70f2235f406a86ed7b8&amp;showmsg=1'>etc</a>/<a href='/guix/log/etc/completion?id=ce451cdcb8f36ca48d4fe70f2235f406a86ed7b8&amp;showmsg=1'>completion</a>/<a href='/guix/log/etc/completion/zsh?id=ce451cdcb8f36ca48d4fe70f2235f406a86ed7b8&amp;showmsg=1'>zsh</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/etc/completion/zsh?id=ce451cdcb8f36ca48d4fe70f2235f406a86ed7b8'>Collapse</a>)</th><th class='left'>Author</th></tr>
<tr class='logheader'><td><span title='2021-10-24 20:17:05 +0200'>2021-10-24</span></td><td class='logsubject'><a href='/guix/commit/etc/completion/zsh?id=55f6d9e818512b4e3e4adacfa99deb201b6854be'>etc: completion: Filter gratuitous spaces from available packages.</a></td><td>Liliana Marie Prikler</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
Tested with:

  guix environment --{ad-hoc,pure} zsh guix bash-completion coreutils -- zsh
  % fpath=(~/guix/etc/completion/zsh $fpath)
  % autoload -U compinit; compinit

* etc/completion/zsh/_guix (_guix_list_available_packages): Remove spaces.

Co-authored-by: Tobias Geerinckx-Rice &lt;me@tobias.gr&gt;


</td></tr>
<tr class='logheader'><td><span title='2021-08-28 17:57:50 +0200'>2021-08-28</span></td><td class='logsubject'><a href='/guix/commit/etc/completion/zsh?id=a8dd285d5a0670abf124a721e6ba94da045b24ba'>zsh completion: Add missing commands and arguments.</a></td><td>Noah Evans</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
* etc/completion/zsh/_guix (_guix_install, _guix_remove, _guix_upgrade):
  New functions.
  (_guix_build, _guix_challenge, _guix_download, _guix_environment,
  _guix_packages, _guix_gc, _guix_hash, _guix_lint, _guix_package,
  _guix_publish, _guix_pull, _guix_refresh, _guix_size, _guix_system):
  Add some of the missing arguments.
  (_guix_cmds): Add missing commands.

Co-authored-by: Brice Waegeneire &lt;brice@waegenei.re&gt;


</td></tr>
<tr class='logheader'><td><span title='2021-02-17 10:52:28 +0100'>2021-02-17</span></td><td class='logsubject'><a href='/guix/commit/etc/completion/zsh?id=ee2a5da80a9bda25542c00a7a35a9ddddcbd58af'>scripts: system: Remove 'vm-image' command.</a></td><td>Mathieu Othacehe</td></tr>
<tr class='nohover-highlight'><td/><td colspan='3' class='logmsg'>
Remove the 'vm-image' command that has been superseded by the 'image'
command.

* gnu/system/vm.scm (system-qemu-image): Remove it.
* guix/scripts/system.scm (system-derivation-for-action): Mark 'vm-image'
command as deprecated and use the image API to produce the VM image.
(perform-action, show-help): Adapt accordingly.
* tests/guix-system.sh: Ditto.
* doc/guix.texi (Invoking guix system,
Running Guix in a VM): Ditto.
* etc/completion/fish/guix.fish: Ditto.
* etc/completion/zsh/_guix: Ditto.


</td></tr>