Three different ways to use a CMake project in another CMake project

[lyrahgames]

In general there should be three ways to use a CMake project inside another project.

• By using find_package together with target_link_libraries for system-wide installed packages
• By using ExternalProject_Add with some extra stuff for frequently changing lightweight projects with lots of different versions that do not have to use CMake
• By using something like a git submodule and a call to target_link_libraries in the build-tree

In my opinion a project should always make sure that every option is possible. For now, I have implemented the first approach. The third one should be easy. I have not tested the second one. We should even consider to test every possibility with our CI environment.

There should be even a possibility to create packages with different components for linux package managers. I think 'CPack' should be one of the tools we have to think about.

[lyrahgames]

The CMake package installation and exportation is implemented and tested by CircleCI. But it should still be tested with Windows and MacOS. It is also possible to uninstall the library. Additionally, I have added the installation of a pkgconfig-file.