Update Doxygen.cmake

Check if the settings we set are already set by the user, and if so don't overwrite them.

Author: Subtixx

src/Doxygen.cmake

@@ -12,13 +12,25 @@ function(enable_doxygen DOXYGEN_THEME)
   if(NOT ${_is_verbose})
     set(DOXYGEN_QUIET YES)
   endif()
-  set(DOXYGEN_CALLER_GRAPH YES)
-  set(DOXYGEN_CALL_GRAPH YES)
-  set(DOXYGEN_EXTRACT_ALL YES)
-  set(DOXYGEN_GENERATE_TREEVIEW YES)
+  if(NOT DEFINED DOXYGEN_CALLER_GRAPH)
+    set(DOXYGEN_CALLER_GRAPH YES)
+  endif()
+  if(NOT DEFINED DOXYGEN_CALL_GRAPH)
+    set(DOXYGEN_CALL_GRAPH YES)
+  endif()
+  if(NOT DEFINED DOXYGEN_EXTRACT_ALL)
+    set(DOXYGEN_EXTRACT_ALL YES)
+  endif()
+  if(NOT DEFINED DOXYGEN_GENERATE_TREEVIEW)
+    set(DOXYGEN_GENERATE_TREEVIEW YES)
+  endif()
   # svg files are much smaller than jpeg and png, and yet they have higher quality
-  set(DOXYGEN_DOT_IMAGE_FORMAT svg)
-  set(DOXYGEN_DOT_TRANSPARENT YES)
+  if(NOT DEFINED DOXYGEN_DOT_IMAGE_FORMAT)
+    set(DOXYGEN_DOT_IMAGE_FORMAT svg)
+  endif()
+  if(NOT DEFINED DOXYGEN_DOT_TRANSPARENT)
+    set(DOXYGEN_DOT_TRANSPARENT YES)
+  endif()
 
   # If not specified, exclude the vcpkg files and the files CMake downloads under _deps (like project_options)
   if(NOT DOXYGEN_EXCLUDE_PATTERNS)