vim_configurable: misc improvements

- fix wrongly used *native* build inputs;
- remove confusing `prePatch = "cd src";` ;
- adapt RPATH handling to multiple-output changes;
- don't list full compiler flags in vim --version,
  as that would keep references to -dev paths.

Together, the closure of the default feature-set drops almost by 100 MB.
The lean vim attribute would *not* lose any references due to patching
--version, so we only apply it for vim_configurable.