Fix emulators/hatari pre-emptively for CMake 3.11. Build error is

CMake Error at /usr/local/share/cmake/Modules/CheckIncludeFiles.cmake:63 (message):

Unknown arguments:

  HAVE_SDL_CONFIG_H

which comes because SDL_INCLUDE_DIR can contain more than one element,
in which case the code falls over. Further explanation is in the patch.

Approved by: tcberner (mentor, implicit)