Allow compilation with --enable-cray.

This patch allows compilation of slurm with --enable-cray without build errors,
on such (multi-cluster) systems that need to be able to load select/cray, but
without actually using the internal functionality (e.g. slurmdbd host).

The code stubs have been translated into corresponding comments with FIXMEs
added, these are then addressed later in the (post-2.2) patch set.

Added the missing part (variables) to fix build errors and tested tested that 
the compilation of --enable-cray works without errors.