Avoid warning about sys/sysctl.h on glibc-2.30.

Avoid warning about sys/sysctl.h on glibc-2.30.

From glibc-2.30 NEWS:

	The Linux-specific <sys/sysctl.h> header and the sysctl function have
	been deprecated and will be removed from a future version of glibc.

Compilation against the glibc-2.30 headers results in the following warning:

	In file included from ipc.c:10:
	/usr/include/sys/sysctl.h:21:2: warning: #warning "The <sys/sysctl.h> header is deprecated and will be removed." [-Wcpp]

On NetBSD and FreeBSD, however, we still need to include the header
to get the declaration of sysctlbyname(3).

This patch changes ipc.c to include sys/sysctl.h only if __FreeBSD__ or
__NetBSD__ is defined.  Also remove the pointless check for __APPLE__.