cmake-mode.el: Use line-beginning-position instead of obsolete point-at-bol

That fixes Emacs 29 warning:

    cmake-mode.el:186:48: Warning: ‘point-at-bol’ is an obsolete function
      (as of 29.1); use ‘line-beginning-position’ or ‘pos-bol’ instead.

Auxiliary/cmake-mode.el

@@ -183,7 +183,7 @@ set the path with these commands:
   )
 
 (defun cmake-point-in-indendation ()
-  (string-match "^[ \\t]*$" (buffer-substring (point-at-bol) (point))))
+  (string-match "^[ \\t]*$" (buffer-substring (line-beginning-position) (point))))
 
 (defun cmake-indent-line-to (column)
   "Indent the current line to COLUMN.