Extend CMake Project to support `make install` and `find_package()`

[thorsten-klein]

In order to build and install matheval the cmake project should use install() statements in order to support make install, which installs all relevant files.
Additionally matheval should also provide some cmake EXPORT so that it can be searched by a customer cmake project e.g. via find_package(boost.matheval)

[hmenke]

Yes, I agree. I should adapt the build system to use CMakePackageConfigHelpers. My TODO list is pretty long at the moment, but I'll come back to it.

[thorsten-klein]

Hello,
thank you for your fast response 👍🏻
If I find time I will contribute and create some PR.