Disable target dox when defined by external project

CMakeLists.txt

@@ -94,7 +94,7 @@ ENDIF()
 
 # -- Doxygen (https://vicrucann.github.io/tutorials/quick-cmake-doxygen/)
 FIND_PACKAGE(Doxygen) # check if Doxygen is installed
-IF(DOXYGEN_FOUND)
+IF(DOXYGEN_FOUND AND NOT TARGET dox)
     # set input and output files
     SET(DOXYGEN_IN ${PROJECT_SOURCE_DIR}/Doxyfile.in)
     SET(DOXYGEN_OUT ${PROJECT_BINARY_DIR}/Doxyfile)