Add lookup-documentation handler for cmake-mode

Closes #4277

Co-authored-by: Sam Whitlock <samwhitlock@users.noreply.github.com>

modules/lang/cc/autoload.el

@@ -156,6 +156,18 @@ the children of class at point."
                          do (push (cons (1+ depth) child) tree)))))))
     (eglot--error "Hierarchy unavailable")))
 
+;;;###autoload
+(defun +cc-cmake-lookup-documentation-fn (_)
+  "Look up the symbol at point in CMake's documentation."
+  (condition-case _
+      (progn
+        (save-window-excursion (cmake-help))
+        (when-let (buf (get-buffer "*CMake Help*"))
+          (pop-to-buffer buf)
+          t))
+    (error nil)))
+
+
 ;;
 ;; Hooks
 

modules/lang/cc/config.el

@@ -154,7 +154,10 @@ This is ignored by ccls.")
 ;; Major modes
 
 (after! cmake-mode
-  (set-docsets! 'cmake-mode "CMake"))
+  (set-docsets! 'cmake-mode "CMake")
+  (set-popup-rule! "^\\*CMake Help\\*" :size 0.4 :ttl t)
+  (set-lookup-handlers! 'cmake-mode
+    :documentation '+cc-cmake-lookup-documentation-fn))
 
 (use-package! company-cmake  ; for `cmake-mode'
   :when (featurep! :completion company)