build-sys: always check for setns() and unshare() libs functions

 $ ./configure --disable-unshare
 $ make nsenter

  CC       sys-utils/nsenter.o
In file included from sys-utils/nsenter.c:36:0:
./include/namespace.h:31:19: error: static declaration of ‘unshare’ follows non-static declaration
In file included from /usr/include/sched.h:42:0,
                 from sys-utils/nsenter.c:23:
/usr/include/bits/sched.h:86:12: note: previous declaration of ‘unshare’ was here
make: *** [sys-utils/nsenter.o] Error 1

We have to always check for the libc function independently on
--disable-{unshare,nsenter} to avoid collision between local and libc
declarations.

Reported-by: "Gabor Z. Papp" <gzp@papp.hu>
Signed-off-by: Karel Zak <kzak@redhat.com>