Ubuntu 14.04 LTS now uses glib-2.0 >= 2.40.0, so we no longer need
to support older version which required us to have some preprocessor
checks. This patch gets rid of the code which was needed to support
older versions and adds a min version check for glib in the
CMakeLists.txt

Reviewed-by: Alexandre Viau <alexandre.viau@savoirfairelinux.com>
Change-Id: Ia11f675d592f34996ca8cb8c770dae67ded68842