component/glibmm: add a patch for gcc 8 compatibility

Fix build errors on gcc 8:

   ../glibmm/threads.h: In member function 'GPrivate* Glib::Threads::Private<T>::gobj()':
   ../glibmm/threads.h:661:29: error: cannot convert 'GPrivate' {aka '_GPrivate'} to 'GPrivate*' {aka '_GPrivate*'} in return
      GPrivate* gobj() { return gobject_; }
                                ^~~~~~~~

This backport fixes from
https://github.com/GNOME/glibmm/commit/37d57ae9572b7d74aa385a30313eceae7f2d3fce
plus changes to generated files, as found in
https://gitweb.gentoo.org/repo/gentoo.git/commit/?id=85e6e0e04ae19670b071820947d941e521d1200e
or
https://src.fedoraproject.org/rpms/glibmm24/c/ecc4276486706c0ebc55aff3ac592499dc9df2a9?branch=master

component/glibmm/buildout.cfg

@@ -4,6 +4,7 @@ extends =
   ../glib/buildout.cfg
   ../libsigc/buildout.cfg
   ../perl/buildout.cfg
+  ../patch/buildout.cfg
   ../pkgconfig/buildout.cfg
   ../xz-utils/buildout.cfg
 parts =
@@ -17,8 +18,10 @@ md5sum = e7416beff6cba1f38c2bccd0dc8c3278
 pkg_config_depends = ${glib:location}/lib/pkgconfig:${libsigc:location}/lib/pkgconfig:${pcre:location}/lib/pkgconfig
 configure-options =
   --disable-documentation
+patches = ${:_profile_base_location_}/glibmm-fix-threads-gobject.patch#75ab0e9e5d1df55e8f6177b9934359ae
+patch-options = -p1
 environment =
-  PATH=${perl:location}/bin:${pkgconfig:location}/bin:${xz-utils:location}/bin:${glib:location}/bin:%(PATH)s
+  PATH=${patch:location}/bin:${perl:location}/bin:${pkgconfig:location}/bin:${xz-utils:location}/bin:${glib:location}/bin:%(PATH)s
   PKG_CONFIG_PATH=${:pkg_config_depends}
   CPPFLAGS=-I${gettext:location}/include
   LDFLAGS=-L${gettext:location}/lib -Wl,-rpath=${gettext:location}/lib

component/glibmm/glibmm-fix-threads-gobject.patch

+From 37d57ae9572b7d74aa385a30313eceae7f2d3fce Mon Sep 17 00:00:00 2001
+From: Kjell Ahlstedt <kjellahlstedt@gmail.com>
+Date: Wed, 20 Dec 2017 20:00:32 +0100
+Subject: [PATCH] Glib::Threads::Private: Fix gobj()
+
+Bug 791711
+---
+ glib/src/threads.hg | 2 +-
+ 1 file changed, 1 insertion(+), 1 deletion(-)
+
+diff --git a/glib/src/threads.hg b/glib/src/threads.hg
+index 86d7a17b..c82a6130 100644
+--- a/glib/src/threads.hg
++++ b/glib/src/threads.hg
+@@ -628,7 +628,7 @@ public:
+    */
+   inline void replace(T* data);
+ 
+-  GPrivate* gobj() { return gobject_; }
++  GPrivate* gobj() { return &gobject_; }
+ 
+ private:
+   GPrivate gobject_;
+--- a/glib/glibmm/threads.h	2017-09-04 15:27:31.000000000 +0200
++++ b/glib/glibmm/threads.h	2018-05-05 10:53:44.339288554 +0200
+@@ -657,7 +657,7 @@
+    */
+   inline void replace(T* data);
+ 
+-  GPrivate* gobj() { return gobject_; }
++  GPrivate* gobj() { return &gobject_; }
+ 
+ private:
+   GPrivate gobject_;