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.

[t20100]

If field is None, it returns a dict which contains the build config for the static lib which is passed to BuildCLib setuptools command.
If field is provided, it returns the value of the field. It is called with way from the hdf5 plugin code using the compression lib to get what needs to be passed for linking with the compression lib to the setuptools Extension. Extension expects lists for 'extra_link_args' and 'libraries'.

It cannot be all stored in a dict since 'extra_link_args' and 'libraries' are not expected by BuildCLib.
It would probably be best to use a class/dataclass/namedtuple here, or a function to extract what's relevant to BuildCLib. Here I've minimised the rewrite, but I agree it makes things complex

[t20100]

Let's merge PR #335 first