vim: Build with system python on Xcode-only Macs

Also end the crazy loads of options.
Note, that if both `--with-foo` and `--without-foo` are defined as
options, then `build.with? 'foo'` does not work as expected for
`:recommended` options.

Fixes #21825
neko 2.0.0-6ab8f48

Closes #20329.

Signed-off-by: Samuel John <github@SamuelJohn.de>
neko: Fix path for libneko.dylib

`@executable_path` in the binaries was
incorrect. so moving to lib/neko/libneko.dylib
fixes this.