chunk 1:Fix build with gtksourceview>=3.8.0

chunk 2:Fix for -fno-common, from
https://gitlab.gnome.org/GNOME/giggle/-/commit/57fd690279c4f8f0a367ec4f3599ab3a8159be49

Index: src/giggle-diff-view.h
--- src/giggle-diff-view.h.orig
+++ src/giggle-diff-view.h
@@ -21,8 +21,7 @@
 
 #include <glib-object.h>
 #include <gtk/gtk.h>
-#include <gtksourceview/gtksourceview.h>
-#include <gtksourceview/gtksourcelanguagemanager.h>
+#include <gtksourceview/gtksource.h>
 
 #include "libgiggle/giggle-revision.h"
 
@@ -35,7 +34,7 @@ G_BEGIN_DECLS
 #define GIGGLE_IS_DIFF_VIEW_CLASS(klass) (G_TYPE_CHECK_CLASS_TYPE ((klass), GIGGLE_TYPE_DIFF_VIEW))
 #define GIGGLE_DIFF_VIEW_GET_CLASS(obj)  (G_TYPE_INSTANCE_GET_CLASS ((obj), GIGGLE_TYPE_DIFF_VIEW, GiggleDiffViewClass))
 
-enum {
+typedef enum {
 	STYLE_CHUNK,
 	STYLE_FILE,
 	STYLE_ALL,