Improve the build system with cmake's ExrternalProject_Add

Currently, the build instructions rely on packagest from the Linux distribution, which is fine. However, the other libraries xStudio depends on (OpenEXR, etc) are to be built manually and installed in /usr/local. This is a *terrible* practice as it will lead to all sort of library misshaps as soon as there are library updates.
cmake provides an instruction called ExternalProject_Add() to download, build and (locally) install the libraries as part of the cmake project.
Please make use of that.

Start with the build instructions and the CMake project configuration, then review how OpenEXR and the other xStudio dependencies are currently built and installed. The work is done when ExternalProject_Add() downloads, builds, and locally installs those dependencies as part of the CMake build, without requiring manual installation in /usr/local.

Written by the indexing model from the issue text.