cleanup header file including to remove external dependencies
Merge branch 'master' of https://github.com/roever/STLL
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).