Don't hard-code the SDL library name
Remove usage of HAVE_SDL for external programs that use STLL

It doesn't make sense to force programs using STLL to define HAVE_SDL.
Whoever includes layouterSDL.h does want to use SDL.  There's no
ambiguity here to solve.

This change ensures that HAVE_SDL remains only for the internal STLL
implementation (layouterSDL.cpp).
Merge pull request #6 from vog/remove-external-have-sdl-dependency

Remove usage of HAVE_SDL for external programs that use STLL
Merge pull request #7 from vog/dont-hardcode-sdl-library-name

Don't hard-code the SDL library name