build: Ignore -Wcast-function-type in G_DEFINE_TYPE

Workaround for https://bugzilla.gnome.org/show_bug.cgi?id=793175
This ignores the warning for G++ 8 and later (which is apparently when it
was added.)

Closes #117.

gjs/context.cpp

@@ -116,7 +116,15 @@ struct _GjsContextClass {
     GObjectClass parent;
 };
 
+/* Temporary workaround for https://bugzilla.gnome.org/show_bug.cgi?id=793175 */
+#if __GNUC__ >= 8
+_Pragma("GCC diagnostic push")
+_Pragma("GCC diagnostic ignored \"-Wcast-function-type\"")
+#endif
 G_DEFINE_TYPE(GjsContext, gjs_context, G_TYPE_OBJECT);
+#if __GNUC__ >= 8
+_Pragma("GCC diagnostic pop")
+#endif
 
 enum {
     PROP_0,

gjs/coverage.cpp

@@ -46,9 +46,16 @@ typedef struct {
     GFile *output_dir;
 } GjsCoveragePrivate;
 
+#if __GNUC__ >= 8
+_Pragma("GCC diagnostic push")
+_Pragma("GCC diagnostic ignored \"-Wcast-function-type\"")
+#endif
 G_DEFINE_TYPE_WITH_PRIVATE(GjsCoverage,
                            gjs_coverage,
                            G_TYPE_OBJECT)
+#if __GNUC__ >= 8
+_Pragma("GCC diagnostic pop")
+#endif
 
 enum {
     PROP_0,

libgjs-private/gjs-gdbus-wrapper.cpp

@@ -30,7 +30,15 @@ struct _GjsDBusImplementationPrivate {
     guint                 idle_id;
 };
 
+/* Temporary workaround for https://bugzilla.gnome.org/show_bug.cgi?id=793175 */
+#if __GNUC__ >= 8
+_Pragma("GCC diagnostic push")
+_Pragma("GCC diagnostic ignored \"-Wcast-function-type\"")
+#endif
 G_DEFINE_TYPE(GjsDBusImplementation, gjs_dbus_implementation, G_TYPE_DBUS_INTERFACE_SKELETON)
+#if __GNUC__ >= 8
+_Pragma("GCC diagnostic pop")
+#endif
 
 static void
 gjs_dbus_implementation_method_call(GDBusConnection       *connection,