One cannot #include <sys/sysctl.h> before <sys/sysinfo.h> on
Linux because <sys/sysctl.h> in glibc-2.3.2 #defines _LINUX_KERNEL_H
which causes the inclusion of <sys/sysinfo.h> to be skipped.
The following program demonstrates:

#include <sys/sysctl.h>
#include <sys/sysinfo.h>

int
main(int argc, char* argv[])
{
        struct sysinfo si;

        sysinfo(&si);

        return 0;
}
}