BUG: ITK unable to detect number of threads under Unix in recent glibc.

_SC_NPROCESSORS_ONLN is defined in unistd.h. It appears this used to be implicitly defined by stdlib by glibc but this is no longer the case.

Change-Id: I442c2df910c7e00235ec0bc84eac669714ea8f23
Merge topic 'pthreads_unistd'

d2af490 BUG: ITK unable to detect number of threads under Unix in recent glibc.