cmake-mode.el: Add support for prog-mode hooks (#15521)

Since version 24, Emacs supports a generic mode called prog-mode.  Like
all other modes it has its own mode-hook, prog-mode-hook.  For Emacs
users it is common to provide all your generic programming-mode related
customizations in this mode-hook.

cmake-mode is definitely a programming-mode and should support calling
this hook. There are two ways to make that happen:

* Make your major-mode a derived-mode from prog-mode.
* Manually calling the hook upon mode-activation.

Implementing a derived mode may be the most proper thing to do, but that
may require quite a few structural changes.  For now just call the hook
explicitly if it exists.  This should cover much of what users need.

Auxiliary/cmake-mode.el

@@ -259,7 +259,9 @@ the indentation.  Otherwise it retains the same position on the line"
   (setq comment-start "#")
 
   ; Run user hooks.
-  (run-hooks 'cmake-mode-hook))
+  (if (boundp 'prog-mode-hook)
+      (run-hooks 'prog-mode-hook 'cmake-mode-hook)
+    (run-hooks 'cmake-mode-hook)))
 
 ; Help mode starts here