Remove stray __USE_GNU macro definitions.

Causes build failures on Debian ia64 platform, and per the glibc docs this
is an internal macro for them that should be getting set automatically
as long as we've defined _GNU_SOURCE before including our first glibc
header.

Fix the task/affinity plugin code to define _GNU_SOURCE first, and not
defer this to the affinity.h include (which may come after some glibc
headers have been included, leading to issues finding the CPU_SETSIZE
macro).

Bug 7152.