inkscape fails to build due to glibmm inlining, regression caused by glib2 changes

[jeremyhu (Jeremy Huddleston Sequoia)]

Updating to glib2-2.48.2 caused inkscape (or any project that uses parts of glibmm) to fail to build because gthread.h now obfuscates struct _GThread.

In file included from ui/dialog/new-from-template.cpp:12:
In file included from ui/dialog/new-from-template.h:14:
In file included from /opt/local/include/gtkmm-2.4/gtkmm/dialog.h:31:
In file included from /opt/local/include/gtkmm-2.4/gtkmm/box.h:59:
In file included from /opt/local/include/gtkmm-2.4/gtkmm/container.h:30:
In file included from /opt/local/include/gtkmm-2.4/gtkmm/widget.h:48:
In file included from /opt/local/include/gtkmm-2.4/gtkmm/rc.h:31:
In file included from /opt/local/include/gtkmm-2.4/gtkmm/style.h:46:
In file included from /opt/local/include/gtkmm-2.4/gtkmm/iconset.h:35:
In file included from /opt/local/include/gtkmm-2.4/gtkmm/stockid.h:26:
In file included from /opt/local/include/glibmm-2.4/glibmm.h:89:
/opt/local/include/glibmm-2.4/glibmm/threads.h:209:11: error: field has incomplete type 'GThread' (aka '_GThread')
  GThread gobject_;
          ^
/opt/local/include/glib-2.0/glib/gthread.h:49:16: note: forward declaration of '_GThread'
typedef struct _GThread         GThread;
               ^
1 error generated.