Support {Net,Open}BSD OSS

Fixed this build failure building raylib:
http://www.cpantesters.org/cpan/report/a069fade-0e1f-11e8-a1cf-bb670eaac09d

Functionality is untested, but it now compiles on NetBSD. A quick google search for
libossaudio OpenBSD indicates that linking to libossaudio would be required there too.

raylib issue: raysan5/raylib#463

Fixes issue #7

This issue affects the OpenSL and SDL backends.

This commit implements a generic method for determining the name which
simply performs an enumeration and compares against IDs. The problem
with this technique is that it cannot know the name of the default
device without help from the backend.

 * Remove dependency on SDL.h where possible
 * Use run-time linking where possible
 * Support both SDL 1.2 and SDL 2
 * Add device enumeration when using SDL 2

The Emscripten build currently uses SDL 1.2 because it avoids the need
for "-s USE_SDL=2" when compiling. This may change later if there's
ever a need or if there's enough demand.