Replace FetchContent_MakeAvailable()

FetchContent_MakeAvailable() was introduced in CMake 3.14.  Use
FetchContent_GetProperties() and FetchContent_Populate() instead as
per the CMake FetchContent documentation:

  https://cmake.org/cmake/help/latest/module/FetchContent.html

This change also bumps the required version of CMake from 3.5 to 3.11.