Handle pstack like other optional directories
  Check if a directory exists relative to $srcdir


Makefile.am:
  Handle pstack like other optional directories
  Check if a directory exists relative to $srcdir
configure.in:
  Handle pstack like other optional directories
  Check if a directory exists relative to $srcdir
pstack/Makefile.am:
  Handle pstack like other optional directories
  Check if a directory exists relative to $srcdir