Aux: update vim-syntax-highlighting

This commit updates the vim-syntax-file. It is still not complete, but contains much more keywords,
built-in variables. Generator expressions are highlighted as well.

This file is generated from a script which parses the --help
output of cmake 3.5.

Imported from here: https://github.com/pboettch/vim-cmake-syntax



Signed-off-by: Patrick Boettcher <patrick.boettcher@posteo.de>