Fix:

error: templates must have C++ linkage

https://gitlab.gnome.org/GNOME/glib/-/issues/2331
https://src.fedoraproject.org/rpms/gconfmm26/blob/4e74a371eef957bbd581cc0501ef110bb7861794/f/gconfmm26-2.28.3-no-extern-c-glib-includes.patch
---  gconf/gconfmm/callback.h	2011-10-18 11:33:49.000000000 -0400
+++  gconf/gconfmm/callback.h	2021-02-13 09:21:06.167235506 -0500
@@ -22,10 +22,7 @@
 #ifndef GCONFMM_CALLBACK_H
 #define GCONFMM_CALLBACK_H
 
-extern "C"
-{
 #include <gconf/gconf-client.h>
-}
 
 #include <glibmm/object.h>
 #include <gconfmm/entry.h>
diff -Naur  gconf/gconfmm/schema.h  gconf/gconfmm/schema.h
---  gconf/gconfmm/schema.h	2011-10-21 04:51:27.000000000 -0400
+++  gconf/gconfmm/schema.h	2021-02-13 09:21:24.616354041 -0500
@@ -31,10 +31,8 @@
 #include <gconfmm/value.h>
 
 
-extern "C" {
     // TODO: do not include this.
 #include <gconf/gconf-schema.h>
-}
 
 namespace Gnome
 {
diff -Naur  gconf/src/schema.hg  gconf/src/schema.hg
---  gconf/src/schema.hg	2011-10-18 11:33:49.000000000 -0400
+++  gconf/src/schema.hg	2021-02-13 09:21:58.002568546 -0500
@@ -24,10 +24,8 @@
 
 _DEFS(gconfmm,gconf)
 
-extern "C" {
     // TODO: do not include this.
 #include <gconf/gconf-schema.h>
-}
 
 namespace Gnome
 {