BUILD: cfgparse: always defined _GNU_SOURCE for sched.h and crypt.h

BUILD: cfgparse: always defined _GNU_SOURCE for sched.h and crypt.h

_GNU_SOURCE used to be defined only when USE_LIBCRYPT was set. It's also
needed for sched_setaffinity() to be exported. As a side effect, when
USE_LIBCRYPT is not set, a warning is emitted, as Ilya found and reported
in issue #1815. Let's just define _GNU_SOURCE regardless of USE_LIBCRYPT,
and also explicitly add sched.h, as right now it appears to be inherited
from one of the other includes.

This should be backported to 2.4.