select/cray: add linking dependency for libjob.so

This adds a link test for libjob.so
 * salloc needs direct support (Makefile.am),
 * however, X_AC_SGI_JOB comes much later in configure.ac.

(An alternative would be, since the libjob interface has practically not
 changed for 2.6 kernels, to integrate its 19 ioctls into slurm.)