Fix unnecessary/restrictive public target flags

[nyoma-diamond]

Resolves #3125, where building dlib using one compiler or system and linking to a consumer using a different one is incompatible.

Key changes:

• Changed some calls to target_compile_options and target_compile_features from PUBLIC to PRIVATE, preventing dlib's compile-time configuration forcing incorrect assumptions on the consumer.
  • active_compile_opts
  • dlib_needed_public_cflags (now named dlib_needed_private_cflags)
  • dlib_needed_public_ldflags (now named dlib_needed_private_ldflags)
• Combined active_compile_opts and active_compile_opts_private (the latter no longer needs to be a separate variable)
• Applying active_compile_opts no longer requires different methodologies depending on if MSVC + CMake < 3.11 are used (simpler approach)