[Usage] using inside cmake project

[yiakwy]

Good job! But I still have a question to use it in CMAKE project.

Suppose I have project in the following structures

- root
      - CMakeLists.txt
      - cmake
      - modules
      ...
      - vendor
            - variant

How should I include the variant libraries for codes implemented in modules?

[yiakwy]

Moreover, I ran bench and found that boost::variant was averagely faster than mapbox::variant with better data.