Add --enable-deprecated and put it in DISTCHECK_CONFIGURE_FLAGS

The actual flag that causes the problem with Gstatic*Mutex is -DG_DISABLE_DEPRECATED. I followed what pluma did but then without that flag.

I added a config option that we can enable so that we can compile it on newer glib. I also put it in DISTCHECK_CONFIGURE_FLAGS so that make distcheck completes.

Do let me know which you like to have added to the list of deprecations when we enable deprecations. Or poke me on IRC to make further changes.