Build: Added HDF5PLUGIN_SYSTEM_LIBRARIES env. var. to build using system libraries

[t20100]

This PR reworks the management of C libraries in setup.py to:

• Allow to use system libraries instead of embedded sources, closes Add a way to build with system libraries #324
• Run embedded plugins/libraries configuration code only if they actually are needed, related to [5.0.0][debian] IndexError: list index out of range #328
• Fix link with dynamic system libs instead of static ones, closes hdf5plugin build against dynamic system libraries instead of embedded static ones #332

[t20100]

Tested with conda: no longer link with available system libs if not requested + use them if requested through the HDF5PLUGIN_SYSTEM_LIBRARIES env. var. (the corresponding pkg-config .pc file must be available).

[t20100]

setuptools is really not appropriate for building pure-C/C++ dynamic libs such as the hdf5 filter plugins.

[vasole]

I have only found things like this:

https://stackoverflow.com/questions/42585210/extending-setuptools-extension-to-use-cmake-in-setup-py

[payno]

'side question' not sure I get this part. Why is this returning a list ? I would expect an empty dict.