Add support for earlier versions of GLib

Added some macros to allow gstdashdemux to compile and operate on
earlier versions of GLib. Tested against GLib version 0.28.6 and 0.34.0,
this will make it easier to use in a buildroot based environment such as
an STB.
Merge pull request #23 from DougGore/master

Added support for earlier versions of GLib