Add a way to build with system libraries

[t20100]

First raised in conda-forge/hdf5plugin-feedstock#55

The idea is to allow using system libraries for the compression libs instead of the embedded ones.
The request comes from conda-forge, but OpenSUSE is also using this approach through a patch.

I propose to use a HDF5PLUGIN_SYSTEM_LIBS environment variable that sets the list of system libs to use (the value can also be all) as it is done for the HDF5PLUGIN_STRIP env. var.