Fixes #3440: Move header check fix from trunk to v1.7

---svn-pre-commit-ignore-below---

r27715 [[BR]]
On OpenBSD, both sys/mount.h and sys/sysctl.h require sys/param.h in order
to compile.  With recent Autoconf versions, AC_CHECK_HEADERS only succeeds
if the header compiles, so need to check these two with sys/param.h
explicitly included in the search list...