make MPR library available to mHM

We need to make the MPR library available to mHM. This should be done using native CMake commands (e.g. FetchContent or CMake CPM. Ideally we try to find a system where MPR is searched by find_package and if not found, it falls back to cloning and compiling and linking the repository itself.

In a first step, as find_package will not work, we make the cloning and compiling approach work. Later (and optionally), MPR thus needs to be available as a (static) library and there should be an install script (so find_package works).

Ideally, but not necessarily, MPR is in version 1.0.