port diskfree detection in fix halt for MacOSX

/* ----------------------------------------------------------------------

   determine available disk space, if supported. Return -1 if not.

------------------------------------------------------------------------- */

#if defined(__linux)

#if defined(__linux) || defined(__APPLE__)

#include <sys/statvfs.h>

#endif

double FixHalt::diskfree()

{

#if defined(__linux)

#if defined(__linux) || defined(__APPLE__)

  struct statvfs fs;

  double disk_free = -1.0;

  if (dlimit_path) {

    disk_free = 1.0e100;

    int rv = statvfs(dlimit_path,&fs);

    if (rv == 0)

    if (rv == 0) {

#if defined(__linux)

      disk_free = fs.f_bavail*fs.f_bsize/1048576.0;

    else

#elif defined(__APPLE__)

      disk_free = fs.f_bavail*fs.f_frsize/1048576.0;

#endif

    } else

      disk_free = -1.0;

    MPI_Bcast(&disk_free,1,MPI_DOUBLE,0,world);