-Wno-error=deprecated-declarations persists due to GtkConfigStore

This is a move of issue https://www.nsnam.org/bugzilla/show_bug.cgi?id=2978

Short summary: We have been adding -Wno-error=deprecated-declarations as part of CXXFLAGS (src/config-store/wscript line 40) to work around numerous GTK deprecations in the libraries.

It was planned as a temporary workaround but we are now two years into it. We probably need to research whether GTK has improved (not sure it has) since then, and if not, whether we keep this or whether we remove support for GTK config store (is anyone using it?).