All the `#if` stuff could be isolated in a single module (vte.c), and not even spread out as much as the current runtime `dlopen()` support (we currently check at compile-time _and_ runtime for much stuff).

BTW, I'm currently tinkering with geany-vte.m4 to try and handle all the VTE versions including with and without gtk3, and whether `--enable-vte` was passed yes or auto, it's a combinatorial explosion, but it would make the code nicer/easier.