Trim vendored dependency trees

third_party/magnum/src/CMakeLists.txt

@@ -42,9 +42,13 @@ set_directory_properties(PROPERTIES
 
 add_subdirectory(MagnumExternal)
 add_subdirectory(Magnum)
-add_subdirectory(MagnumPlugins)
+if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/MagnumPlugins/CMakeLists.txt")
+    add_subdirectory(MagnumPlugins)
+endif()
 
 # Magnum include dir for superprojects
 set(MAGNUM_INCLUDE_DIR ${CMAKE_CURRENT_SOURCE_DIR} CACHE INTERNAL "" FORCE)
 
-add_subdirectory(debuggers)
+if(EXISTS "${CMAKE_CURRENT_SOURCE_DIR}/debuggers/CMakeLists.txt")
+    add_subdirectory(debuggers)
+endif()