fixing cmake warnings about deprecated function

[jaja360]

When doing the command cmake -B build ., I got four different warnings saying:

  Calling FetchContent_Populate(cxxopts) is deprecated, call
  FetchContent_MakeAvailable(cxxopts) instead.  Policy CMP0169 can be set to
  OLD to allow FetchContent_Populate(cxxopts) to be called directly for now,
  but the ability to call it with declared details will be removed completely
  in a future version.

This PR fixes three of these by updating the CPM dependency, and fixes the fourth warning by modifying dependencies/CMakeLists.txt:27 manually.

[lemire]

Let us merge.