Fix some autoconf issues on OpenBSD

Fix some autoconf issues on OpenBSD

Issue 1: autoconf gets accept when a header works properly with cpp
but not with cc.  This was true of the sys/sysctl.h header on
openbsd.  The fix: include sys/param.h (if present) when testing for
sys/sysctl.h

Issue 2: Somehow, autoconf's macro generation code is messed up on
some versions of openbsd (including mine, and other people's too) so
that instead of SIZEOF_VOID_P, it makes SIZEOF_VOID__.
evutil/util.h now works around that.