Adds bpy.app.sdl to expose SDL version information.
When SDL is not available on a Linux system, certain Blender features are silently disabled (like joystick support in the BGE). This change is the first step towards making it more obvious why something isn't working. For example T43647 was simply solved by installing SDL 2.0, but Blender didn't give any indication that this would be necessary.
SDL information is exposed to Python via bpy.app.sdl, in the same way as OCIO and OIIO information is exposed.
Generated system-info.txt contains SDL loading method (dynamically linked or dynamically loaded) and SDL version number.