cmake-mode.el: syntax of '_' should be treated as symbol

Word commands, such as foward-word(M-f), backward-kill-word(M-backspace),
don't work well like other major-modes if syntax of '_' is treated as "word".

Tested-by: Guillaume Papin <guillaume.papin@parrot.com>
Merge topic 'emacs-mode-underscore-in-symbol' into next

30f14aeb cmake-mode.el: syntax of '_' should be treated as symbol