Enhancing cmake documentation/implementation

Here are a few points, which should be changed/extended in the current cmake/README.md

• It should be mentioned right from the start in the document (including command line examples), that Ninja is required to build octopus
• It would be helpful, if examples are given how to link a specific version of a library. For example an older and a newer version of libxc (not the default preset ones), that one woul like to compare for debugging purposes
• It would also be instructive to have explicit examples, how to set own compiler flags for the employed Fortran and C/C++ compilers
• For non-cmake experts, it would also be helpful to see (with explicit command line examples) how to enable/disable certain libraries

For the Octopus webpage, we should have build instructions for the most popular operating systems. This should include the OS packages that are needed, as well as explicit command line examples for the build.

closed by !2606 (merged)