GTK3 port: add '--with-gtk=3' to the configure script.

GTK3 port: add '--with-gtk=3' to the configure script.

After the last few changes, the whole codebase now compiles and links
successfully against GTK3, and I can run an experimental pterm. The
config box and font selector look ugly, but the basics all seem to
work.

In order to compile at all, I had to manually bodge in the extra
compile flag -Wno-deprecated-declarations. My plan is to fix all the
uses of deprecated things, and then remove that flag.

I've made GTK3 the second choice, after GTK2 but before GTK1. GTK2 is
the only GTK version that produces a completely sensible build (partly
because the GTK3 port is visibly unfinished, and mostly because its
server-side font handling is just too slow), so it remains the first
choice.