Avoid compiler error in ibus_engine_dbus_property_changed

Do not return a value in a function that returns void.
Target GLib 2.36 for Vala if it is installed

Avoid a compiler warning when g_type_init() is called on GLib version of
2.35.0 or above by targetting GLib 2.36.0 when generating C code with
Vala.
Check for gsettings-schema-convert for dconf support