feat(build): HAVE_UNIBILIUM

compile time features are hot again.

Note: this changes the &term value for builtin definition from
'builtin_xterm' to just 'xterm'. It's an xterm regardless of we use an
external definition or an internal. Prior to this commit the vast
majority of POSIX users will have used external terminfo, so plugins and
scripts are only going to have checked for &term == 'xterm' or 'tmux' or
whatever.
The status of external loading is still available in "nvim -V3" output.

runtime/doc/vim_diff.txt

@@ -473,8 +473,9 @@ TUI:
   - Note: If you want to detect when Nvim is running in a terminal, use
     `has('gui_running')` |has()| or see |nvim_list_uis()| for an example of
     how to inspect the UI channel.
-- "builtin_x" means one of the |builtin-terms| was chosen, because the expected
-  terminfo file was not found on the system.
+- Nvim might optionally be compiled with unibilium, in which case the terminfo
+  database will be used. Otherwise, or if the terminal was not found in
+  the database, a table of builtin terminal definitions will be used.
 - Nvim will use 256-colour capability on Linux virtual terminals.  Vim uses
   only 8 colours plus bright foreground on Linux VTs.
 - Vim combines what is in its |builtin-terms| with what it reads from terminfo,