build-sys: check for union semun instead of using _SEM_SEMUN_UNDEFINED

build-sys: check for union semun instead of using _SEM_SEMUN_UNDEFINED

Older versions of glibc used to declare ‘union semun’ in <sys/sem.h>,
but POSIX.1-2001 requires the caller to declare it instead. Later
versions of glibc started defining _SEM_SEMUN_UNDEFINED to note that
the union was not being declared, but conforming systems are not
required to define that macro (e.g. FreeBSD). As a side effect we get
rid of some obsolete __GNU_LIBRARY__ macro usage.

[kzak@redhat.com: - use #ifndef]

Signed-off-by: Guillem Jover <guillem@hadrons.org>
Signed-off-by: Karel Zak <kzak@redhat.com>