Add back in previously removed handling of deprecation flags
* Restores the deprecation flag handling in configure script but adds an
additional option for disabling deprecation prohibition flags even when
gtkpod version is a development build.
* ./configure --enable-deprecations=yes will stop the addition of all the
deprecation prohibition flags hence allow deprecated function calls
* ./configure --enable-deprecations=no will ensure that the deprecation
flags will be added to development builds