gconfmm: Fix build with glib 2.67.3 and later

Closes: https://trac.macports.org/ticket/69964

gnome/gconfmm/Portfile

@@ -19,7 +19,7 @@ long_description \
 maintainers     nomaintainer
 categories      gnome
 license         LGPL-2.1+
-homepage        https://www.gtkmm.org/
+homepage        https://gitlab.gnome.org/Archive/${name}
 master_sites    gnome:sources/${name}/${branch}/
 use_xz          yes
 
@@ -29,7 +29,8 @@ depends_build   path:bin/pkg-config:pkgconfig \
 depends_lib     port:glibmm \
                 port:gconf
 
-patchfiles      patch-configure.ac.diff
+patchfiles      patch-configure.ac.diff \
+                gconfmm26-2.28.3-no-extern-c-glib-includes.patch
 
 compiler.cxx_standard   2011
 

gnome/gconfmm/files/gconfmm26-2.28.3-no-extern-c-glib-includes.patch

@@ -0,0 +1,47 @@
+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
+ {