Support dynamic loading of SDL libraries

This is mainly to address old issue when one need to have SDL library installed
in order to use our official builds. Some hip distros already installs SDL,
but it's not quite the same across all the variety of the distros.

We also now switching to SDL-2.0, most of the distros have it in repositories
already, so it shouldn't be huge deal to install it if needed.

Reviewers: campbellbarton

Reviewed By: campbellbarton

Differential Revision: https://developer.blender.org/D878