Added extra check in for setaffinity support.

The Linux version of tMPI_Thread_setaffinity_support() did not check
whether pthread_setaffinity_np() would actually work on the system
it's running on. It now checks by running pthread_getaffinity_np()
and checking its return value.

This is relevant for, for example, Bluegene systems which don't
support explicit affinity setting.

Change-Id: I4833e7384067f897013d6fa4d0b35963ae740fcf