configure.ac: Don't create Makefiles for sdlnet and none.

This fixes the following errors when generating and running configure in
a fresh repository:

configure.ac:301: error: required file
  'src/multiplayer/sdlnet/Makefile.in' not found
configure.ac:301: error: required file
  'src/multiplayer/none/Makefile.in' not found
config.status: error: cannot find input file: `Makefile.in'