ncurses requires a proper TERM env variable to compile, and fails on docs.rs

With the big 6.0 refactor, the constants generation done at build time now requires a proper `TERM` variable _at compile time_. While this is generally available when building from a terminal, a lot of building happens without a terminal (from an IDE, in CI, ...). In these environment, ncurses currently fails to build properly. Builds on github CI require manually injecting `TERM=xterm` in the builder environment for all users of this library. For other environments like docs.rs, there is no clear way to make it build (which is why https://docs.rs/ncurses still shows the last version that still built, 5.101.0). It would be a good idea to inject this `TERM=xterm` during the build process if no `TERM` is set.