Add CMake support

[JohelEGP]

Greetings.

I made this because I need to use this library, and CMake is the easiest way for me to consume it. I hope this is well-received.

A few notes:

• CMake 3.8.0 is required to use cxx_std_17. The library's introduction says it uses "idiomatic C++11/14," but I quickly met build errors because it uses std::string_view.
• CMake has no built-in module to find PCRE, so I unconditionally link against pcre2-8. One of the many FindPCRE.cmake in the web could be used instead for better portability, minding their copyright and licenses of accompanying projects.
• With CMake, it would be possible to automate some of the things described in the "Building Unicorn" section of the library's introduction, among others, like regenerating unit-test.cpp when adding new tests.