On some systems <sys/sysinfo.h> may be present while the sysinfo
function may not, or at least not as part of standard libraries,
so check whether the function is actually available before using
it.

This fixes builds on FreeBSD with the devel/libsysinfo present.
Signed-off-by: Gerald Pfeifer <gerald@pfeifer.com>
Signed-off-by: Alexandre Julliard <julliard@winehq.org>