Avoid exporting compile options

[ken-matsui]

Problem:

• The ut (aliased as Boost::ut) target exports compile options as INTERFACE, but it mostly does not need. (this is also pointed out as FIXME)
• However, it seems that tests, examples, and benchmarks want to use the options.

Solution:

• Delete compile options for the interface ut.
• Add compile options for targets of tests, examples, and benchmarks as PRIVATE.

Issue: #

Reviewers: @

[ken-matsui]

Hi @krzysztof-jusiak, Could you please review my PR at your convenience?