Why does external library adds compiler flag -std=gnuc++11 in cmake

I am trying to add external library VTK in my project using cmake. What I have noticed is when I try to add this external library it silently add -std=gnu++11 but when I remove this third party library that flag does not get added. Only flag I add is -std=c++17.

How I add external library is :

tell cmake the place where I have installed vtk

list(APPEND CMAKE_PREFIX_PATH ${CMAKE_CURRENT_SOURCE_DIR}/vtk)

.. now link with vtk library

TARGET_link_libraries( ${target} ${VTK_LIRARIES} )