Panic if libgtk-3 appears to be built with enable-debug=no
Panic if libgtk-3 appears to be built with enable-debug=no

Safety of this crate's API relies on `g_return_if_fail` checks being
enabled. Refuse to work with library built with `enable-debug=no`,
which exposes undefined behavior all over the place.
Kudos to @efyang for discovering ways to implement this check.
Close #270.

EPashkin/rust-gnome-gtk,rust-gnome/gtk,daa84/gtk,Susurrus/gtk,rust-gnome/gtk,daa84/gtk,Susurrus/gtk,EPashkin/rust-gnome-gtk