Disable deprecation warnings from Glib.

Having glib spew out pages of warnings just because we happened to
build on a machine with a newer version installed, telling us to modify
perfectly working code to break compatibility and risk regressions for
no reason...  seems a bit daft.

Fortunately we can simply define a macro to shut it all up.

Doing this in config.h revealed that many source files don't include it,
which is a bug in its own right so fix that up at the same time.