[CMake] Only build vtkdiff when OGS_BUILD_TESTS=ON.

ThirdParty/CMakeLists.txt

@@ -32,7 +32,7 @@ if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/tetgen/CMakeLists.txt)
 endif()
 
 # vtkdiff
-if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/vtkdiff/CMakeLists.txt)
+if(EXISTS ${CMAKE_CURRENT_SOURCE_DIR}/vtkdiff/CMakeLists.txt AND OGS_BUILD_TESTS)
     add_subdirectory(vtkdiff)
     install (PROGRAMS $<TARGET_FILE:vtkdiff> DESTINATION bin COMPONENT ogs_extras)
 endif()