It would be good, error reporting-wise, to be able to signal which
function should be used instead of a deprecated one. GCC 4.5 added an
optional "message" payload to the deprecated attribute, so that:

  void f1 (void) __attribute__((deprecated("Use f2 instead")));

Will expand to:

  warning: f1 is deprecated: Use f2 instead

Instead of just printing:

  warning: f1 is deprecated

Since we already have a G_GNUC_DEPRECATED macro we should provide a
G_GNUC_DEPRECATED_FOR macro defined as:

  G_GNUC_DEPRECATED_FOR(bar)

Which would expand the deprecation message to "Use bar instead"
automatically. The deprecation message should probably be similar
to what we use in gtk-doc to match up with the documentation.

https://bugzilla.gnome.org/show_bug.cgi?id=614965